The best place to farm Frostweave cloth is just north from Icecrown Citadel, at least for cloth users. You can find many groups of Converted Heroes here and everyone has a chance of dropping up to 4 Frostweave Cloth. They are immune to movement impairing effects, stuns and what not. They also slow you down so it will be hard to pull more than 3 packs at once.
Let's spice it up a bit!
Should a plate user want to farm, they will have a lot more fun doing this, since they can go to the Cathedral of Darkness instead, sitting next to Icecrown Citadel. Level 85 recommended, as they are all elites and if you pull one, 10 more will follow. They respawn super fast so you don't have to run around searching for next mobs.
Tip: I highly recommend tailoring profession for this one, especially if you're high enough to learn Northern cloth scavenging skill, which makes every humanoid drop more Frostweave cloth.
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