Open Educational Resources (OER)

...the challenge is finding exactly what you're looking for.  Why? Because with more than 20 years of OER creation, there's LOTS of it out there. And remember, the definition of OER includes everything that is no longer protected by copyright, everything that has gone into the public domain.  And all the openly-licensed academic books out there (even if not textbooks per se.) Talk about too much of a good thing!

Fortunately, you have several options for how to search for OER. Depending on your preference, you can use a metasearch engine (searches multiple websites), look at well-established, popular repositories, or one of the referatories (they list and link out to the OER rather than archive it themselves).

• Launches a real-time, simultaneous search across 22 different sources of open educational materials, including the major, long-standing OER repositories, open publisher's sites, and directories.
• Results are most up-to-date
• It searches several sources of public domain materials...this can at times clutter your search results with older materials
  • To avoid this, uncheck the 'Deeper Search' option on the advanced search page

Mason OER Metafinder (MOM)

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Open Textbook Library

BC Campus OpenEd

Merlot Open Textbooks Collection

LibreTexts

OER Commons

Milne Open Textbooks (formerly OpenSUNY)

OpenStax 

PressBooks Collections