One way is to go to Driftveil City and go to the coast with a pacing man on it. Then, surf east until you get to the lighthouse. The stone is on the rock somewhere. You may need the Dowsing Machine.
Vaperon i think.
route 6
water stone
It evolves with a water stone
you evole it with a water stone
A Staryu evolves with a water stone.
Use Surf in Driftveil and surf right. The iitem on the rocks is a Water Stone
it evolves into Simipour by using a water stone.
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 light stone isn't in Pokemon white only the dark stone in white and the light stone in black
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!
there is a guy on a deck in castelia city. he will ask you what monkey Pokemon you got from the dream yard. if you say the wrong Pokemon you will get the wrong stone.