A few large sites:
Open Textbook Initiative (American Institute of Mathematics )
Open Course Libraries (Washington state),
UBC site (University of British Columbia).
Links to many more resources:
Research Guides: Discovering Open Educational Resources (OER): Home.
Temple University Libraries guide to alternative textbook projects.
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Faculty grant programs: In 2011, the University of Massachusetts Amherst launched the Open Education Initiative (OEI), a competitive grant program to help faculty members make the switch to open textbooks in their classrooms. After disbursing 40 grants of approximately $1,000 each over the course of four years, the OEI projected student savings at almost $1.5 million.
http://opentextbookalliance.org/wp-content/uploads/2016/08/Open-Textbook-Alliance-Guide.pdf