Where to get dusk balls in white veriosn?

You can't get Dusk Balls in Pokemon White. They were introduced in Pokemon Black 2 and White 2.

However, you can get similar balls in Pokemon White:

* Premier Balls: You get 10 Premier Balls for completing the game (after beating the Elite Four and becoming Champion). You can also buy them from the Game Corner in Castelia City for 1000 points each.

* Luxury Balls: These can be bought from the Poke Mart in Undella Bay for 2000 Pokedollars each.

If you're looking for Dusk Balls specifically, you'll need to play Pokemon Black 2 or White 2.