was told that I am supposed to get a spell at level 75, after completing a quest. But i don't know where to get the spell from!
Go to Wizard Central and look at the spells for whatever school you are (you didn't tell us) and see if you do have a quest, and where to find it. There's no other way for anyone to know without knowing what school you are. sorry
You need to have completed The Runedown quest (level 55 spell quest) and the Crab Alley side story quests with King Thermidor ending with Coming up for Air. Once you complete those, Prof Balestrom give you the quest to talk to the Professor that sends you looking for the spell.
give you the quest to talk to the Professor that sends you looking for the spell.