Does anybody know where you can buy a Scroll of Windwalker ???

500 pt levitate for 60 sec and Invisibility for 60 sec

Quite simple really... you can't

They're found as random loot in some level dependant bandit cave barrels. Usually ones with lots of scrolls. Probably about level 25+ would be enough to find one.

I think there are two fixed ones, one in a Daedric ruin ?Anudnabia - certainly one where flying helps and the other in a dunmer stronghold. Unfortunately as I never use them, I never record which.