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.
From UESP wiki, "First Lessons" quest article:
If you do not have the spell [Faralda] asks of you, she will offer to teach it to you for 30 gold.
Talk to Faralda and choose the appropriate conversation option to buy the 'Conjure Flame Atronach' spell. You should now be able to use it.
Also, from the same UESP article linked above:
Alternatively, you can attempt to persuade her and avoid this test, or if you are here for the quest Elder Knowledge, you can tell her you are Dragonborn after which she will ask to see a shout. Either of these will get you in without needing to cast a spell. In order to persuade her, you will need a Speech skill of 100 (70 with the Persuasion perk) or an Amulet of Articulation, available only by completing the Thieves Guild questline and becoming Guildmaster.
Best Answer
The game can be played any way you want, but some of the advantages of visiting the College of Winterhold are:
It is a long way, but you can easily travel there using a wagon. If you're following the main quest line, once you get to Whiterun you can take a wagon from in front of the stables.
So, I would recommend traveling there shortly after you reach Whiterun. After getting Lydia or after visiting the Throat of the World seem like good times to peel off and do the mage quests.