Frostweave drops everywhere for me. Not sure I would bother farming it. I thought that doing the "Plague Cauldron" daily in Icecrown non-stop might work (I assume you can keep doing it after you complete it???). Maybe in Old Strat? I dunno though. It just seems to drop a lot all over for me. If it were me, I would just do dailies instead and you will collect a bunch over time.