Go to the Celadon City department store and buy one.
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Weepinbell will evolve when you use a leaf stone on it.
You have to get an Oddish first, then evolve it into Gloom. After that you give Gloom a Leaf Stone and he will evolve into Vileplume.
The leaf stone will evolve certain species of pokemon
You get a leaf stone underground.
Arcanine is the evolved form of Growlithe. Neither of them can be caught in the wild on Leaf Green. Growlithe is a Pokemon Fire Red exclusive pokemon. To be able to evolve it, give it a Fire Stone and then trade it to your Leaf Green game.