How can one obtain university level textbooks from Canada if your not signed up for classes?

It’s not the dark ages anymore… people should be able to get up-to-date materials from universities without paying $10,000 a year just to get the right to buy textbooks from them…my provincial library system mostly only has pop culture science books… What happened to the advanced math books, the astrophysics… where can a Canadian get textbooks that are master’s level or higher without signing up for classes….E-bay doesn’t have what I’m looking for either.