Frostweave can only be found in Northrend and can be dropped by most humanoids from level 70 to 80.The best instance to find Frostweave in WoW would have to be Cathedral of Darkness in Icecrown. If you have AoE spells you will have no problems taking down 24 level 80 elites on one side and by the time you’re done with one side the other side with the exactly the same mobs would have re-spawned. This cycle is great for farming Frostweave in WoW. You will also be looting plenty of gold and other items that can be vendored.
See below for the location to farm Frostweave Cloth in WoW.