This Frostweave Cloth farming guide will show you the places I use for farming Frostweave Cloth in Wrath of the Lich King Classic.
All three of these places below are great farming places, I recommend trying out all 3 and then farm at the one you prefer.
Every humanoid around Onslaught Harbour can drop Frostweave Cloth. You can do pretty big pulls if you have good gear, but many of them are ranged, so this is best for DoT classes.
It doesn't matter how fast you can kill Mjordin Combatant at this spot, this area was coded so that there are always at least 3 of them present at one time so they will respawn immediately.
Farming Bonescythe Ravagers in Sholazar Basin is also a good farming place. But, make sure you are farming at the marked spot, if you farm too close to the big elite mob there, she will one-shot your mobs, and you won't be able to loot them.
These mobs cast Bone Saw which does a lot of damage.