I need one to evolve my Misdreavus.I know that you get one from Lenora after you get the dragon skull back from Team Plasma,but I used it already...
The one on Route 10 is rather difficult to find, here is the way starting from the Opelucid City side gate. Go towards Victory Road, but stop when you find the dark grass. From there go through the dark grass and follow its path until you find a Male Veteran. Talk to him and you receive a Dusk Stone. He also gives you advice on how to evolve Lampent ^_^
You can find the Dusk stone on Route 10, in Mistralton Cave, and in dust clouds. I hope I helped.
Lenora gives you the Moon Stone.