You can evolve gloom to vileplume,and 2nd evolution of belsprout into victricbel I'm not sure about victribel.I don't know any more. Sorry about my spelling I'm Serbian.
You get a leaf stone underground.
You can't I've tried looking but there in no leaf stone
The leaf stone will evolve certain species of pokemon
to evolve gloom you need a leaf stone or a sun stone. sun stone=Bellossom leaf stone=Vileplume you can get a sun stone from a person in the space station and a leaf stone on route 119. hope i helped :)
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.