I need a dusk stone to evolve lampent but can't find one. This is for pokemon WHITE version
On Route 10, a man near the northwestern corner of the map will give you one just for making it up there.
If you need more than one you can always use repel and farm in caves you might just end up with a dusk stone .
One more in mistralton cave on 3F you will find one dusk stone.
go to the same cave where you find cobalion, its in a pokeball near it.