There may be other ways to get a Leaf Stone in Pokemon White, but here is the easiest way to do it if you haven't already:
1.Go to Castelia City.
2.Go to the far right of the city, to the far right pier.
3.Go to the pier, and there will be a person there. Talk to him, and he will ask you which Pokemon you got towards the beginning- Panpour, Pansear, or Pansage.
4. Answer Pansage and he will give you a Leaf Stone.
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-MrEvan340
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.
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.
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 will evolve certain species of pokemon
You get a leaf stone underground.
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!
Pokemon
It doesn't evolve by level, but rather, by giving it a Leaf Stone.
Gloom and Weepinbell.I think there are some more Pokemon that evole with a leaf stone but I forgot....