I hit 100 in Destruction and went to the College of Winterhold to see about master spells but I don't see any on the Destruction spell NPC (can't remember her name). All she has are spells up to Expert level. Where can I find master level destruction spellbooks?

You have to talk to her, you should see the option "Is there anything else to know about Destrution Magic?" or something to that effect. Then she gives you a book, and you start a quest.

Its the same with each of the trainers, once you hit 100 in a magic school you talk to one of the masters and they wil give you a ritual quest which rewards the player with acess to master level spells.