First, once you get to Hearthome City, go to Bebe's house and she will give you a free Eevee if you enough space. Then find a Thunder Stone Underground ,or somewhere else, and evolve Eevee into a Jolteon!
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Jolteon does not evolve, but to get a Jolteon you need a thunderstone to evolve an Eevee into Jolteon.
Jolteon
On route 214, There is a trainer with a Jolteon, Flareon and a Vaporeon. The trainers name is collector Douglas
i have Pokemon soul-silver and i evolved my evee using a thunderstone
a collector on route 212 named Douglas has a jolteon flareon and a vaporeon hope this helped : )