I killed the High Dragon at the Mountain Top of Haven, he has LOTS of items (mostly armor) to loot, among those is Dragon Skin. I take it to Master Wade's Emporium in Denerim and he says he needs Drake skin to practice before he can craft Dragon skin.
If I remember correctly the drakes were in the cave where you met the Kolgrim the reaver and his band that wanted to talk to you. The drakes were in a side tunnel that lead to a cave room with cages and as I remember 6 drakes appeared, enough for Wade to practice his armor making skills.
Any other method of acquiring drake scales is unknown to me.