Hi doeflax, welcome to MR!
You'll find thousands of cheap--in fact, free!--books
right here on MR. Of course, these are all in the public domain. So, if you like to read classics, you could be set for years!
I'll let others answer for the best places to buy non-public domain books, because I haven't bought any books from other than Amazon.
You won't find any discussion on MR about obtaining books illegally.