Unlike the Pokemon game or the anime, you can’t just buy or find an element stone and evolve your Eevee in Pokemon Go but it looks like some fans did manage to find a way to get the Eevee to evolve the way they want it to.
The way to evolve Eevee in Pokemon Go right now is to feed it the candy and pray that it end up evolving into what you want. However, a group of Reddit users seems to have come out with a way to get the Eevee to evolve to whichever one they want.
Since the Pokemon available in Pokemon GO right now is the first generation Pokemon, Eevee can only evolve into Flareon, Vaporeon, and Jolteon. According to the Reddit post, you can decide what it will become by nicknaming it Sparky, Rainer or Pyro.
Sparky will give you Jolteon, while Rainer will you Vaporeon. Pyro will end up being Flareon. Those are names of the Eevee brothers in the Pokemon series.