Where do you get a sun stone in Pokemon black version?

There is no "Sun Stone" in Pokémon Black version. Sun Stones are used to evolve certain Pokémon into their final forms, and they are not available in Pokémon Black.

However, there are similar evolution items in Pokémon Black:

* Moon Stone: Found in the Relic Passage, a cave located near the entrance to Undella Bay.

* Fire Stone: Located in the Giant Chasm, the area leading to the Pokémon League.

* Thunder Stone: Found at the beginning of the route leading to the Pokémon League.

If you're looking to evolve a Pokémon that requires a Sun Stone, you'll need to trade with someone who has Pokémon White, as Sun Stones are only found in that version.