You cannot buy leaf stones you have to earn them from people like lennora the second gym leader or find them in poke balls.
Pansage will evolve by using a Leaf Stone into Simisage.
The only Unova Pokemon that evolves with a Leaf Stone is Pansage.
Pansage
Go to black city and gain people from pokemon white's white forest who gives you the option of selling leaf stone. After, save, go to the market that just appeared in your city and buy (expensive) leaf stones.
Petilil will evolve with a sun stone, pansage with a leaf stone, panpour with a water stone, and pansear will evolve with a fire stone.
The Leaf Stone can only make 5 pokemon evolve. Gloom into Vileplume, Weepingbell into Victreebel, Exeggcute into Exeggcutor, Nuzleaf into Shiftry, and Pansage into Simisage.
So far (although I have completed White not yet Black) all I know are Pansage (Leaf Stone), Pansear (Fire Stone) and Panpour (Water Stone). Sorry if this isn't as helpful as you hoped!
pansage
route 6
hm im not sure
give it a leaf stone
Evee->Leafeon.Sorry. Its Pansage.Also Sunkern in white forest/black city.There may be others so experiment =)It's Always an anserbold italics done by me, Chao141