There are an overwhelming number of places to search for OER. The Mason OER Metafinder (below) does a real-time, simultaneous search across 21 different sources. It can take a bit longer than searches of indexed, curated content; however, the results are up-to-the-minute for each search target. Its wide scope makes it a good place to start. For a more advanced search interface and to see which sites are being searched, visit mom.gmu.edu
Mason OER Metafinder (MOM)
The sites listed below are included in a MOM search, but are worth mentioning on their own.
For supplementary material, common sites like YouTube often have a search filter for CC license, making it easy to find free, reusable content.
If you are an instructor serious about adopting OER into your course(s), don't hesitate to ask a librarian for help finding what you need.
High-quality, peer-reviewed, openly licensed college textbooks that are free online and low cost in print. OpenStax also includes free ancillary materials for students and instructors, like homework and review questions. A number of OpenStax textbooks have already been adopted into courses at UM-Flint.
OpenStax teamed up with OER Commons, one of the most popular OER repositories. This Hub is a place for instructors to share ancillary materials that they have created to pair with OpenStax textbooks. OER Commons as a whole (oercommons.org) is a great resource for supplemental materials.
From the University of Minnesota, Open Textbook Library provides a growing catalog of free, peer-reviewed, and openly licensed college textbooks. It's easy to search or browse, and you can read faculty reviews of each item.