It would be helpful for me to know what Pokemon game you're referring to. Since the Fire Red and Leaf Green games are forward compatible, and those are the only games I know you can get one from, I will tell you where to find it :) In Celadon City there is a place called Celadon Mansion. There you will meet a nice old lady who will give you tea. Now if you are curious enough, you will find that the mansion has a back entrance. That's an Eevee at level 25.