If Frostweave Cloth is too expensive, or you can’t find any in AH, there’s a great place to farm them, whether your character is alliance or horde.
I went there with my lvl 76 Deathknight, and I was kept really busy and only had time to kill and loot, kill and loot, and occasionally get some healing.
The secret consists of finding one of those sweet spots, where the mobs respawn, and then just stay there and kill.
You can find this sweet spot in Sholazar Bazin, North-East in The Lost Lands at the co-ordinates 64,45 – 44,60.
There you should kill Bonescythe Ravager, who has a drop rate for Frostweave at 57.6%.
Sometimes, the mobs come one at a time, but most of the time, they are two or three to arrive at your spot. You will get a few seconds pause from time to time, but be prepared to do mindless and endless killing.
I got around 250 Frostweave Cloth in an hour, and many green items, gold etc.