Below are a few places to look for Open textbooks. If you can't find the perfect one, consider creating your own by pulling together parts from several textbooks plus your own unique materials.
The Biodiversity Heritage Library makes biodiversity literature openly available to the world as part of a global biodiversity community. BHL also serves as the foundational literature component of the Encyclopedia of Life.
The College Open Textbooks Collaborative, a collection of twenty-nine educational non-profit and for-profit organizations, affiliated with more than 200 colleges, is focused on driving awareness and adoptions of open textbooks to more than 2000 community and other two-year colleges.
Open-access portal for classic, out-of-print titles from the catalog of Cornell University Press, selected specifically because of their high potential for course use.
"The directory is open to all publishers who publish academic, peer reviewed books in Open Access and should contain as many books as possible, provided that these publications are in Open Access and meet academic standards."
"Created to provide better textbook options to students and faculty, who are frustrated with the limitations and costs of traditional textbooks. We kept what works: expert authors, editorial development, peer review and teaching supplements. And changed everything else to give choices and control back to students and educators."
"Our books are published in hardback, paperback, pdf and ebook editions, but they also include a free online edition that can be read via our website, downloaded, reused or embedded anywhere."
Based at Rice University, "Our free textbooks are developed and peer-reviewed by educators to ensure they are readable, accurate, and meet the scope and sequence requirements of your course."
