After you defeat the goldenrod gym leader go to Ecutreak Pokemon center and Bill will show up. After you talk with him go to his house in Goldenrod. Make sure you have an open spot for the eevee because he wont just send it to your PC
HINT: It would be really useful if your Eevee was a female, because if it's a female, you can breed it to get more Eevees so they evolve into it's different evolutions! Before you talk to Bill, save your game. If your Eevee is a male, switch it off and on till you get a female.
Chat with our AI personalities
You have to get it from a friend who has Pearl/Diamond/Platinum
You can't catch them, but Bill gives you one at his house in Goldenrod.
Pikachu into Raichu, and Eevee into Jolteon. I don't recommend evolving Eevee into Jolteon. Espeon is way better.
You can breed it at the daycare.
5,000