This list is not comprehensive, because there are a large amount of OER resources available and new ones are being created every day, but it should give you an idea of the high quality materials that are available. You can also search for materials on Canvas Commons.
A good place to find OER resources is Canvas Commons. You can find complete courses, modules, and other resources already formatted for Canvas. Just be sure to check the licensing of the resource before downloading it. Here is a guide showing how to use Canvas Commons.
Other places:
Here is a link to a guide that shows how to use Canvas Commons. You can find resources here that are ready to be copied into your Canvas Course Space. Be sure to check the copyright!