OER are often found in OER repositories. While there are a few very big repositories (Merlot, BC Campus, Open Textbook Network, etc.), there are many smaller and often specialized repositories. It's helpful to do the same search across several repositories to be sure you are finding all the materials available on your topic. The library has compiled a list of some of the biggest OER repositories in our Research Guide Open Educational Resources (OER) @ SDSU for Faculty.
Google results often cannot indicate the OER status of an item from the main search screen. It is helpful to do a Google search such as Evolutionary Biology AND OER because you will often find pages that will list OER repositories for that topic - in this example the first few results are for scholarly articles and OER repositories and libguides (subject guides created by librarians with lists of resources on a topic).