So, I'm pretty sure she's not dead.
Unless you've already completed the Restoration Ritual Spell Quest (and gotten the two highest level spells from her), she should be flagged as Essential. That is to say, she should be unkillable. I know that I've been trying to for the past 10 minutes, and while she goes to her knees occasionally, she JUST. WILL. NOT. DIE.
So, what this means is one of two things. Either you've been hit by yet another famous Elder Scrolls Bug, and she's fallen through the world or otherwise despawned. While this is not impossible, I'd characterize it as pretty unlikely.
More likely, is the fact that she, and all the other college mages, wander around pretty constantly, and don't really keep to a 'schedule' the way most NPC's do. I once went weeks in game without being able to track down the Illusion trainer, for example. There are two unowned beds in the Tower of Countenance. They don't belong to anyone. It's not the bed that used to be Collete's.
As to where else you can get high level Restoration Spells, you'll find them randomly as leveled loot as you adventure - especially in Necromancer and Vampire lairs - though you'll never be able to gain access to the final two spells without completing the Ritual Spell quest, for which Collette is required.