It is a very complicated process...
First, you must somehow obtain an Oddish. You can catch one, but buying one off eBay would work just fine.
Second, you must take it somewhere quiet. Any cave or forest will do.
Third, you must sacrifice it. Once sacrificed, you must reach your hand into it's little chest and grab it's heart.
Congratulations! You've obtained a Leaf Stone!
go underground and look for them
Exeggcute and Nuzleaf.
Cathch a GLOOM then give it a leaf stone. For a bellosum give it a sun stone.
you can get it by going to route 119 at the tall grass
give it to celio and then mewtwos cave will be unlocked
Vileplume is not found in the wild in Pokemon R/S, you must evolve it from a Gloom by Leaf Stone.
gloom(to get a vileplume), weepinbell(to get a victreebell).
The same way you do in every other generation, evolve Oddish into Gloom, then evolve Gloom with a leaf stone. You can get a leaf stone from the shards guy if you hand him a green shard.
Nuzleaf only evolves by use of a Leaf Stone, so it can evolve at any level.
Yes, the water stone from Pokemon looks like Sapphire.
The leaf stone will evolve certain species of pokemon
You get a leaf stone underground.