Eevee can be found in the back of a building in Celadon City.
ThunderStones may be bought in Celadon City. There is also a free ThunderStone in the Power Plant.
Jolteon is one of the very few Electric-types found in the Kanto games, and certainly the best amongst the sweepers, only rivaled by Magneton in terms of bulk (which, however, sacrifices speed) and Raichu in terms of movepool (but not by much).
Given the many threats that an Electric-type can easily put to rest in this region - especially in the late game, with the likes of Lorelei as well as Lance's Gyarados and Aerodactyl - players are more than likely to want a STAB Thunderbolt user on their team. For the less-than-lucky nuzlockers who found a Voltorb in the Power Plant, rest assured, there is definitely a better option readily available.
Gym #3 - Lt. Surge (Vermilion City, Electric-type): It is unlikely this battle will be on Jolteon's to-do list, since Eevee becomes available much later. If Jolteon must fight here, at any rate, it will work like a charm: its Volt Absorb allows it to take little to no damage from Lt. Surge's Pokémon, stuck with low-power Normal-type moves, and even though Thunderbolt is resisted by them, it will still respectively 2HKO Voltorb, OHKO Pikachu and 3HKO Raichu, providing that Jolteon is at level 30 or higher. Even an Eevee with Secret Power can do work in this gym, achieving the very same results as Jolteon's Thunderbolt.
Rival (Pokémon Tower): Eevee can win against all of the rival's Pokémon except Gyarados by simply spamming Secret Power, which KOs in a variable amount of hits, usually two or three; Gyarados is too powerful because of Thrash, but caution should also be used against Ivysaur's Leech Seed and PoisonPowder combo, and more generally when Eevee's health is low, as most Pokémon on the rival's team 3-4HKO it. Jolteon simply wins by spamming Thunderbolt, instead (against Exeggcute, Bite is actually more powerful).
Giovanni (Rocket Hideout, Ground-type): Eevee is helpless against these Pokémon, but Jolteon does better. Onix and Rhyhorn can be 2HKOed by Bite, and neither of them has any Ground moves. Kangaskhan, however, can KO in three hits with Fake Out and two Mega Punches, and Jolteon's best option is a 3HKO by Thunderbolt, which it will not have the time to use; at best, Jolteon can weaken Kangaskhan, but it will then need to heal or switch out.
Gym #4 - Erika (Celadon City, Grass-type): Both Eevee and Jolteon are at odds against Erika. Eevee is underpowered against her fully evolved Pokémon, whereas Jolteon's best move, Thunderbolt, is resisted; although Thunderbolt and Victreebel's Giga Drain deal more or less the same amount of damage, Giga Drain offers Victreebel recovery, and this will eventually wear Jolteon down and force it to heal repeatedly during the battle. Vileplume is even worse, due to its higher bulk. A combination of Thunder Wave and Bite may cause just enough flinching for the matchup to work, but the strategy is entirely reliant on luck and, with the low damage output Bite has, it would take several turns to pan out. Jolteon can, however, 2HKO Tangela with the move, as Tangela is the less threatening of the group.
Gym #5 - Koga (Fuchsia City, Poison-type): Thunderbolt is guaranteed to OHKO the Koffing and 2HKO Weezing; the latter has no explosive moves, so Jolteon can fight it safely. Muk is only 3HKOed and will make itself harder to hit by using Minimize as well as poisoning Jolteon with Toxic, so Jolteon should preferably avoid it, but it can nevertheless put in some work before switching out if needed.
Fighting Dojo (Saffron City, Fighting-type): Jolteon can win against Hitmonlee, but only with strategy: its Hi Jump Kick is a 2HKO, and Jolteon only 3HKOs. However, it typically uses another move on the first turn, such as Mind Reader; if it does, Jolteon can carry Protect on a temporary basis and use it on the second turn, to get rid of Hitmonlee with two Thunderbolts and Hi Jump Kick recoil. Hitmonchan also 2HKOs, however, and Jolteon has no available workarounds in this case.
Rival (Silph Co.): Thunderbolt OHKOs Pidgeot and Gyarados, 1-2HKOs Growlithe and Charizard, and 2HKOs Blastoise and Alakazam. None of these Pokémon represent a threat for Jolteon, even in the event of critical hits; only Alakazam can, with Calm Mind, but Thunderbolt is highly likely to 2HKO even after one usage and, should it not happen, Alakazam will still be unable to OHKO Jolteon. Exeggcute is 2HKOed by Bite. Venusaur outdamages Jolteon with its Razor Leaf but, if Jolteon has Thunder Wave as well as Bite, it can attempt to stall it out and eventually either heal or switch if the strategy does not work out.
Giovanni (Silph Co., Ground-type): Thunderbolt 2HKOs Nidorino and Kangaskhan without risks, but Rhyhorn and Nidoqueen are harder to beat. Rhyhorn can KO in two turns with a five-hit Rock Blast, and Jolteon's Bite is a 3HKO; although Jolteon can technically win, it is up to the luck of the draw. Nidoqueen can paralyse and 3-4HKO with Body Slam, whereas Jolteon's Bite takes five to six hits to beat it, so it should be fought by a better suited Pokémon if possible.
Gym #6 - Sabrina (Saffron City, Psychic-type): For this fight, having both Thunderbolt and Shadow Ball is useful. Kadabra is OHKOed by Shadow Ball or 2HKOed by Thunderbolt; Mr. Mime is 2HKOed by both. However, Kadabra has Reflect and Mr. Mime has Barrier, and these moves can alter the amount of turns needed to defeat them; Kadabra less so, since with Shadow Ball it will not have the chance to use any given move, though Mr. Mime may survive two Shadow Ball attacks if Barrier is used - in that case, finish it off with Thunderbolt instead. Venomoth falls to two Thunderbolts, as well. Alakazam is 2HKOed by either Thunderbolt or Shadow Ball, but only Shadow Ball maintains 2HKO status even after a Calm Mind, which mutates Thunderbolt into a 3HKO; having at least Shadow Ball to dispose of Alakazam is crucial, as it 2HKOs with Psychic without any boosts.
Gym #7 - Blaine (Cinnabar Island, Fire-type): Thunderbolt 2HKOs Growlithe and Ponyta with an OHKO chance; even in the 2HKO case, their Fire Blasts can only 3-4HKO Jolteon. Rapidash is 2HKOed for certain, and its Fire Blast is also only a 3HKO. Jolteon should not fight Arcanine, as its Fire Blast 2HKOs and Thunderbolt cannot.
Gym #8 - Giovanni (Viridian City, Ground-type): A Jolteon entering this battlefield would only be needlessly sentenced to death. Jolteon should not fight here.
Rival (Route 22, pre-Elite Four): Thunderbolt still destroys Pidgeot, Growlithe and Gyarados in one hit, and Charizard and Blastoise in two. Bite is a 1-2HKO against Exeggcute, but Venusaur is stronger than Jolteon and, if the Thunder Wave and Bite paraflinch does not work, Jolteon should be healed or switched out. Alakazam now has Psychic, so it should not be fought by a wounded Jolteon; a healthy one, instead, can take two hits and it 2HKOs with Thunderbolt. Rhyhorn is also 2HKOed by Bite, though carefulness is once again needed if Jolteon is damaged, as a five-hit Rock Blast can deal up to around 70% to Jolteon.
Elite Four Lorelei (Indigo Plateau, Ice-type): Jolteon OHKOs Dewgong, Cloyster and Slowbro with Thunderbolt, which also 2HKOs Jynx and Lapras effortlessly.
Elite Four Bruno (Indigo Plateau, Fighting-type): Jolteon's Bite is a 2HKO against Onix, and both of them also 2HKO with Earthquake; while Jolteon is by no means their ideal counter, it can work. Thunderbolt is also a 2HKO against Hitmonlee, Hitmonchan and Machamp; Hitmonlee and Hitmonchan can 3HKO at best, though Machamp is riskier, due to its Cross Chop being a 2HKO with an enhanced critical hit rate. Jolteon will obviously not be able to solo Bruno, but has good chances of dealing with about half of his team.
Elite Four Agatha (Indigo Plateau, Poison-type): Thunderbolt is an OHKO against Golbat and Haunter and a 2HKO against the two Gengar and Arbok. The only potential threat is the first Gengar's Confuse Ray; confusion should be healed right away, in order to not impair Jolteon's functioning in this battle.
Elite Four Lance (Indigo Plateau, Dragon-type): Thunderbolt is once again a clean OHKO against Gyarados and Aerodactyl. Although Aerodactyl's Speed is technically tied with Jolteon's, Jolteon should be able to outspeed purely because it will be higher levelled, even if its nature is detrimental to Speed; even if this does not happen, Aerodactyl has no moves that can OHKO Jolteon. Dragonair is 3HKOed and both Outrage and Hyper Beam are also 3HKOs; Jolteon should be safe, since it is faster. Dragonite's Hyper Beam is a 2HKO instead, but due to the recharge turn, Jolteon can defeat that too, providing its health is full when Dragonite comes out.
Champion Rival (Indigo Plateau): Pidgeot and Gyarados are still OHKOed, Charizard and Blastoise are still 2HKOed but cannot do any better. Jolteon and Arcanine both 3HKO each other, but Jolteon will be in the losing end in this case, since Arcanine also knows ExtremeSpeed to finish it off. Bite outdamages Exeggutor's Giga Drain, but it is advised only in tandem with Thunder Wave, as Exeggutor's Light Screen will otherwise turn the matchup upside down; if Exeggutor uses Light Screen, all the turns needed to KO the rival's other Pokémon also double, which is something to keep in mind. Venusaur's only damaging move is SolarBeam, but it also knows Growth and Sunny Day and Jolteon takes far too long to beat it; it should be fought by a properly suited teammate, like Rhydon, which will otherwise murder Jolteon with Earthquake.
Post-Game: Jolteon remains good and reliable throughout, but Mewtwo will laugh at its Bite. It will do better in the second segment of the Sevii Islands instead.
Jolteon's initial movepool is Eevee's fantastic assortment consisting of Helping Hand, Sand-Attack, Growl and Quick Attack. Jolteon will definitely need a Helping Hand to fix that. However, as Eevee learns Bite at level 30 and Baton Pass at 36 and Jolteon does not (it gets Double Kick and Pin Missile instead, respectively), it is highly recommended to wait until that level for the evolution; Eevee is thankfully not too hard to train, with the Secret Power TM available in Celadon City. After evolving, Jolteon sadly does not get much in the way of level-up moves: Thunder Wave at level 42 - which is useful if Jolteon also has Bite, though - then Agility at level 47 which is useful only in Baton Pass sets, and the obligatory Thunder at level 52. Thunder is not terrible, but its accuracy makes it the lesser choice over the much more reliable Thunderbolt, unless Jolteon runs a Rain Dance set.
Only TMs can help Jolteon function properly. Thunderbolt is a must, unless once again Jolteon runs a Rain Dance set with Thunder. If, for some reason, Thunderbolt is not a viable option, or not yet, Shock Wave is a good choice to fill up the move slot until the better move becomes available. It also helps against Koga and other users of moves that cause evasion and accuracy modifiers. Shadow Ball may be an option over Bite, but it is not recommended due to Jolteon's mediocre Attack, unless Shadow Ball itself is being used as a contingency plan against Sabrina, in absence of better options.
Tutors do not offer a lot to Jolteon, with the only viable option being Substitute, and only really useful when paired with Baton Pass and Thunder Wave. However, since Baton Pass can be learned as Eevee, this role is likely more fitting for Vaporeon, as Jolteon's bulk is not great.
Pokémon that are good against Ground-types: Electric-types are trivial to find teammates for, since they have only one weakness. Most Flying-types can provide a good defensive answer to them, though their offensive is lacking and they fall short against Rock/Ground-types; Water-types are typically better, since most Grass-types available in Kanto are also part Poison, thereby losing their Ground resistance.
Physical tanks: With Jolteon's lacking capability of taking physical hits, as well as Ground moves being physical, it is a given that a proper physical tank should supplement it on the team. This works even better if the tank is apt to facing Ground-types.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
What Nature do I want? Anything that boosts Special Attack is a godsend, even Quiet. Jolteon will almost never need that much speed in-game, so losing some of it is not a big deal. The best option is, of course, Modest. Any other nature that detracts from the Attack Jolteon will never ever be using is still good, though, particularly Bold and Calm for some extra bulk.
At what point in the game should I be evolved? Contrary to the other Eeveelutions, Jolteon actually benefits from waiting on the evolution until level 36, as it gets Bite and Baton Pass as Eevee. Neither is present in Jolteon's level-up movepool, and while Baton Pass is entirely optional, Bite adds to Jolteon's otherwise Electric-only special coverage. For non-Baton Pass sets, the recommended evolution level is 30; for Baton Pass sets, it is 36.
How good is Jolteon in a Nuzlocke? Any Electric-type is rather usable by default, but Jolteon is really good. It needs only one TM to properly work, but then again, it is an issue most Electric-types share. Players will never be disappointed by it in a nuzlocke, no matter the game.