There is a book in Stromguarde in Alterac Highlands on the alliance side that should teach you heavy silk if I recall correctly. Then at 225 skill you need to go to Theramore, in the Citadel there is a Dr. guy who will give you the Triage quest to get your skill opened to 300.
He will then teach you Mageweave, at 240 I believe you get heavey Mageweave, and then at 260 you get Runecloth from him.
The quest is a pita, took me about 10 tries to get it right. You have to bandage wounded soldiers in order of the severity of their wounds and you can only let 6 die while saving 15. Best bet is to go 3rd person and angle your camera up a bit so you can swing around the room and pick the worst ones while you are still bandaging that last one.