I think the limiting factor here isn’t a lack of available open source materials. Too many schools have gotten bed with the publishers and are getting a nice cut of the way over priced books. The schools for example could have gone with cheaper publishers without much trouble. They could have told their publishers to not rearrange the same text every year to make it hard on people using used books. Yet, they don’t. Simple fact is that at the end of the day it isn’t in their interests to make it cheaper for you get your books.
Now the community college system is often a different beast. I did the first two years of my degree at the community college in my home state. They stuck with the same books until something significant changed. The campus books store always had a nice stack of used copies of every book you could need and they were the exact same book that was being sold new. When I transferred to a regular school it was entirely different. The books changed every year, sometimes every semester, and many of the professors had written their own supplemental books you had to buy. I wouldn’t be at all surprised if my old community college adopted open source books to save the students money. I suspect hell will freeze over before my university does it.