In Pokemon Black, the Sun stone can be found in/with/at
Dust clouds, Pickup, Nimbasa City, Relic Castle, Black City, held by wild Solrock
Here is a video showing where to get the one in Relic Castle. Here is the one in Nimbasa.
For a complete list of evolution stones and their locations in the game, check out the Bulbapedia page for it, which is where I got the information above.