...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).