The fastest way is to earn points on the Pokemon-a-thon and buy a Leaf Stone there for 2500 points. The Leaf Stone is available at Thursdays I think. You can also get a leaf stone from Bill's Grandpa if you show him an oddish, he is just above Cerulean city. and you can find a leaf stone in viridian forest.
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You may find a leaf stone in Alexa forest or Viridan Forest. It is in one of those places. You will have to look hard for it though.
No such thing. Sent me a message on what you mean. Do you mean a shiny leaf?
use a leaf stone
You can evolve Gloom by a sun stone, or a Leaf stone. Sun stone= Belossom Leaf stone= Vileplume
Get a Leaf Stone and you'll evolve it.