The best place (i think) to collect runeclothes is in Dire Maul. You dont need to go inside any of the instances. Just go inside the buildings. There are a lot of ogres especially in the centre. One of them drops 3-4-5 pieces of runeclothes.
If u have time spend it (1-2 hours) there and will get 100-120 runeclothes. I did it on the Neptulon server with a shadow priest and got about 500 pieces within 2 days. I made it because I could sell it on a very very good price in AH.
Try it if u want.