Buy them used. If it is a brand new version of a text that is just out, check with your tutors to see what version they are using. Often there isn't much added in new versions. And even if there are new chapters added, most tutors won't be jumping to add it to the course material straight away.
And downloading wouldn't be much cheaper than (if at all) buying the book new. If you are suggesting getting it for free, then I would suggest removing that part of your post, doubt TB wants to be promoting such activities

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Another thing you could do is use the university library. But it does depend on the size of your course, how good the library is etc. I've got 11-12 large textbooks (bought over the last 6 years or so), and most of them haven't been used that much. Well, they weren't used that much during my earlier studies, when we were told we needed them. What is it you are studying? Because some fields won't linger on textbooks for long before moving on to focusing specifically on journal articles.