If you go back to Bill's House in Goldenrod City after he's introduced himself at the Pokemon Center, Bill will be there and he'll give you a free Eevee (at level 5) if you talk to him. How can you turn that deal down? Go back and get one right away!
You go to the pokemon centre in goldenrod city and Bill will be there. Make sure you have an empty spot in your party. Go to Bill's house and he will give you an evee.
There's an Evee available as a prize at the Game Corner (you have to play the Voltorb Flip game to get coins). You must have the Coin Case in your Key Items inventory to store coins from the Game Corner. I believe there are two Game Corners in the game: One in Goldenrod City, and another one in a large city in Kanto. An Evee is approx. 5, 000 coins, if memory serves me.
I think we all know by now that bill gives you an eevee but what if you want another; such as a girl for example all my pokemon eggs hatch into fckn boys and I need 3 girls to evolve into vaporeon flameon and glaceon (I can't spell but you know who I mean)