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
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.