Where can i find a dusk stone in pokemon white? 10 points?

im trying to evolve my lampent into i think its called a chandelure . something like that and i really need one . im bout to batte the 8th gym . but right now im in the relic castle battling team plasma and something about a legendary pokemon in there i forgot its name but yea . so anywhere i can find the dusk stone ? thank you sooo much . 10 points . plese help me out .