There is a man in Striaton City, near the entrance to Dreamyard, He is down a passageway that heads south. Talk to that man and you will get a dusk stone.
Here is a list of other places you can get a dusk stone:
Route 10, Mistralton Cave, Wellspring Cave , Chargestone Cave, Mistralton Cave, Twist Mountain, Challenger's Cave, Victory Road , Giant Chasm.
Another option is to get somebody to trade you a pokemon that is holding one.
