Look at the website Gutenberg.org to see if it has any of textbooks you require. You can download 45,000 free ebooks to portable devices such as the iPad, Kindle, Sony Reader, iPhone and Android. From this website, you can also link to over 100,000 ebooks available through the website's partners and affiliates.
Search for your books at BookBooN.com, TextbookRevolution.org, FlatWorldKnowledge.com, FHSST.org and DiplomaGuide.com. Some of the books -- many of which are updated on a regular basis -- appear in PDF form, and others can be viewed online as ebooks. View the catalogs of available titles to find the textbook you need. FlatWorldKnowledge.com, for example, offers ebooks that cover a gamut of subjects including business and economics, professional and applied science, humanities and social science, biology and chemistry. FHSST.org specializes in science textbooks. Professor and teachers have authored many of the textbooks offered by these website, and your high school or university may use some of these titles for instruction.
Choose the textbook you desire, and select the desired format. Some of the websites allow you to read the textbook online while others require you to pay to print the book or download it as an audio book or ebook.
Go to CollegeSwapShop.com to trade textbooks you used during a previous semester for those you will need in a coming semester.
Post an ad on a classified website such as Craigslist.org. Detail which textbooks you need and which you can offer as a trade. Consider offering services such as proofreading or tutoring to trade for textbooks at no cost.