If you’ve already evolved the Eevee from the Celadon Mansion, you will need to breed your Eeveelution with a Ditto to obtain an Eevee Egg which you can evolve once hatched.
Where To Find Eevee
In Pokémon FireRed and LeafGreen, Eevee cannot be found in the wild. Eevee can only be found at the top of the Celadon City Mansion. The good news is that there are not any prerequisites to getting an Eevee, other than being able to walk to Celadon City.
Getting to Celadon Mansion
In order to make it to the top of the Celadon Mansion, you need to make your way behind the building. Head to the northeast side of the city and there will be a gap in the trees. Just pass through the gap and continue west until you’re behind the mansion. Once you’re inside the mansion, keep walking up the stairs. Once you reach the roof, you will see a small room with a roof over it. Walk inside, and take the Pokéball from the table. This Pokéball will have your Eevee in it, and you will be able to nickname it whatever you want. After you have your Eevee, it’s time to get the Thunderstone.
Buying a Thunderstone
Buying a Thunderstone is super convenient because it’s in the same city as Eevee. The Celadon Department Store has most of the evolution stones available for purchase at a reasonable price.
Celadon Department Store
The Celadon Department Store is hard to miss. It’s the large, purple building in the middle of town. Head inside and take the elevator (or stairs, if you’re trying to stay in shape) all the way to the 4th floor. Once you’re on the 4th floor, head down just a little and talk to the lady behind the counter. Here, you can purchase the Thunderstone for 2100 Pokédollars.
Evolving Eevee into Jolteon
Once you have the Thunderstone, it’s very easy to evolve Eevee.
Breeding Eevee (For More Eeveelutions)
If you’ve already evolved the Eevee from Celadon City, then there’s no need to worry because it is possible to get more. However, it will take a lot of time and effort. Before you can breed for a new Eevee, you need to beat the Elite Four, have over 60 Pokémon, and receive the National Pokédex from Professor Oak.
Getting a Ditto
The easiest place to grab a Ditto is on Route 14 to the East of Fuchsia City. There’s one patch of grass on this route blocked by a tree, so you will need a Pokémon that knows the HM Cut. However, once you’re past that tree, Dittos can be found in abundance. Dittos can also be found in these locations:
Cerulean Cave The Pokémon Mansion on Cinnabar Island Route 13
The Four Island Daycare
Getting to Four Island is a bit of a trek, but well worth the journey if you enjoy breeding in general. This will also allow you to breed limitless Eevees and obtain every Eeveelution for your Pokédex. To get to Four Island:
Getting Your Eevee Egg
Once you’ve obtained the Rainbow Pass, head to Four Island and find the Daycare. This Daycare will be able to take care of two Pokémon at once. If you submit two compatible Pokémon, they will create an Egg. To get an Egg that will hatch into an Eevee: After you have your Egg, you will need to keep it in your party for a while as you walk around. Be warned, it will take a lot of walking to hatch your new Eevee. You can check your Egg’s Summary to see how close it is to hatching. Once your Egg is ready to hatch, you will be alerted with a dialog box. After reading through the dialog, an animation will play and you will be greeted by your brand new baby Eevee! Which you can then nickname if you’d like.
