Celtic Heroes

I have been looking for mind tabs of slate for 2 weeks. Now, been camping and killing copp full time, got every single tabs but mind slates! I can understand that these drops are random, but not even a single mind slate after 2 weeks of farming coop?! I have every other items for full meteoric even I have all the items gathered for full frozen, only stuck with 3 mind tab of slates, please help guys! I can even buy them if anyone is selling, but no one is selling them in my server ! Suddenly it became so rare! Please help guys !
Server,
Moban