Textbook exchange here, I've always had really good luck with them! Otherwise sometimes ebay or if it's a new edition I just get it from the bookshop at uni.
If your uni/tafe has an online student portal, check there because students will post lists of textbooks they are selling. Also worth going into the building and looking at noticeboards, as some people will post lists of textbooks they are selling there. Provided you check the edition numbers, you shouldn't have any problems
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