Worn Dragonscales are skinned from the bodies of Dragonkin. Even though these scales aren't pristine and shiny, these tough scales can be used for crafting armor. No dragonkin mobs have a particularly high drop rate, so collecting large stacks requires a considerable amount of grinding. Dustwallow Marsh is reputedly a good place to hunt for them.
You can also occasionally find a stash of dragonscales in Knot Thimblejack's Cache in Dire Maul.