Restoration trainer dead at the college of winterhold. Where can I now buy higher level spells?

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.

Colette Marence FOUND: I had this problem too so i though back to the last time i saw her which was on the bridge in the Eye of Magnus. So after a bit of exploring under the bridge with detect life i found her blue aura stuck trying to walk back to the college but stuck between a set of large rocks. It seems that when she is trying to get past the protective ward she can actually run off the bridge if you don't get it down fast enough.

If this is the case she can be freed by using unrelenting force, by pushing her far enough towards the town(not the college) she will eventually pick up the path back to normal life.

Granted this may not be the answer for every instance but it was mine and i was able to finish the quests for restoration. I hope it helps.