I am LVL 80 now, lvl 100 for all five schools of magic
have not learnt any spell above expert level
(except a fire storm spell aquired from a mage's guild quest)

any one knows where can i buy master spells?

From the college trainers, the others have quests to earn the higher level spells too.

With the lone exception of Guardian Circle, they are sold by the trainer who gives you the master quest. For some reason, I could never buy Guardian Circle from Colette, even after I get Bane of Undead from the Auger.

got all (except illusion) master spell so far

BTW: the conjuction spell flame thrall is really useless, not worth my effort

you mean the trainers will sell master level spells,
in addition to the spells rewarded from the quests from different schools?