Since the Open Educational Resources environment is an ever-growing context, there is no one-way searching strategy.
Still, a useful tool is Major OER Repositories, maintained by Joerdis Weilandt. It is a continuously update Padlet where repositories are shown as on a board.
Also interesting is OER World Map, an interactive tool that, in addition to mapping relevant organisations, projects, people and services, provides social networking, allows the identification of OER collections, and supports decision making (requires the creation of an account for full feature)
Searching through OER repositories might sometimes be overwhelming: this short but effective tutorial, created by Jen Klaudinyi (Faculty Librarian at Portland Community College) will undoubtedly be a great help.
While the many definitions of Open Educational Resources tend to be all-encompassing, one of the most sought after type of OER is the textbook: open textbooks can make the lives of both students and teachers much easier.