If you are in front of the Celadon City Pokemon Center, go right three squares, go up until you run into bushes, go left until you see an opening in the bushes. When you see the opening, go into it. Then, go left unil you see an opening behind a building. Go into that. Then, go up the steps until they stop. Go right, you will run into a wall, then go down. You will see a door, go into it. You will see an Item Ball on a table. Get it. It contains Eevee.