You have to get it from a friend who has Pearl/Diamond/Platinum
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Eevee can't evolve into leafeon in soulsilver you have to trade it to diamond and pearl and evolve it there. The ilex forest doesn't have a Mossy Rock - which is what evolves eevee into leafeon.
You can't leafeon doesn't exist in leafgreen.
No, you cannot evolve Eevee with a leaf stone. It can evolve into the leaf type Pokemon, Leafeon, but it evolves by leveling it up in Eterna Forest in Pokemon Platinum
You do not need friendship to evolve Eevee into Leafeon. All you need to do is level Eevee in Eterna Forest.
Take your Eevee to the Eterna Forest and level it up next to the moss rock.