They are often sold by healers in chapels, and occasionally by spell merchants like Edgar Vautrine and Ungarion. The trick is getting the spell that fortifies what you want fortifying and which is suitable to your expertise in Restoration.
Cast spells
Cast a lot of alteration spells.
You will always have a spell in use on Oblivion. If you don't want to use that spell, then simply select a different one. :)
You cannot construct your own spells as you could in oblivion, you must find or purchase spell tomes and consume them.
The dark elf is very good for spells.
You have to know it as a spell first, find someone to teach you it, then it will be available as an option while enchanting.
It should be in the list of all your other spells. Cast it normally.
30 second invisibility
They are in all of the cities mages guild mostly or in the ARCANE UNIVERSITY.
To kill any kind of ghost or spirit in TES 4: Oblivion, you need to either use spells, or enchanted, silver or daedric weapons.
There isn't one. There is only weakness to fire, frost, shock, magic, and normal weapons spells in oblivion. There is a weakness to disease spell in Morrowind, i think.
My best bet is that you have not saved the game. That's the best I can think of..