Perfect Deviate Scale is a better than average scale from the mutated deviate creatures in the Wailing Caverns. Perfect Deviate Scales are only used in one Leatherworking recipe which is a reward choice for from the Wailing Caverns quest giver in the cave above the Caverns, but the recipe is a superb one, and the individual scales sell for gold in the auction house.
Perfect Deviate Scales are used in the following recipes:
Comments on the linked external sites seem to suggest that the drop rate for this item is better in the entryway than in the instance itself. Some folks have gone through the entire instance, skinning, and gotten only a couple, others have farmed in the entrance area and gotten enough for one belt or two belts in one trip.