When in Celadon city, go up the right side of the Pokemon Center. Follow the path all the way until you get to the back of a building which has a big door and enter. Once you are in there, go up all the stairs until you reach the top level. There, you will find a door with a sign posted on it saying "I know everything." Go in the door, and once inside you will see an item on a table, walk up and pick the item up. Surprise! It's a Poke Ball with Eevee in it! You now have yourself an Eevee. If you would like an Eevee of a specific gender, just save before you pick the ball up. If you don't get the correct gender, just reload your game and try again.
See more: www.youtube.com/watch?v=7TJa33UaGlc
HOPE I HELPED ^_^