Go around the back and follow the stairs all the way up. There will be a small building. Go inside and take the pokeball on the table to get an eevee.
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Eevee is found at the top of the Celadon mansion, the door as it the back of the building. This is the only Eevee.
go to cerulean city and go behind the Pokemon mansion and enter through the back, go all the way to the roof and go into the only room and on the table will be a pokeball with a lvl 25 eevee! Eh-hem. It's CELADON city not cerculen and its the celadon mansion not the Pokemon mansion
you can get the leaf stone, fire stone, and thunder stone at the big department store in celadon city. you get eevee in the back on the Pokemon mansion.
Evolve the Eevee you get given at the top (using the back entrance) of the Pokemon 'Mansion' in Celadon using a Thunderstone.
Eevee does not appear in the wild naturally in Pokemon Sapphire. In order to get one, you have to trade it from either FireRed or LeafGreen, where you can get one in Celadon Mansion, or from Pokemon XD, where you start with one.