Determine the author, title, edition and ISBN number for each textbook you need. You can find this information on the class syllabus or by contacting the professor before classes begin.
Ask your professor if you can use an older edition of the textbook. Publishers issue new editions every couple of years, but textbooks may not change much from one edition to the next. Previous editions of the book may be significantly cheaper than the current edition.
Search by the book title or the ISBN number at such websites as Amazon.com, Half.com, BarnesAndNoble.com, Biblio.com and AbeBooks.com for your textbooks. You may also find books by plugging the ISBN number of each textbook into Google.
Compare prices of books at retailer websites. There will be new and used copies from which to choose, which will enable you to find textbooks in your price range. Do not forget to take into account shipping costs.