You can quickly search OASIS, the Openly Available Sources Integrated Search, from SUNY.
OASIS has collected over 155,000 records from 52 different sources. The sources include the usual textbooks and course materials, but it also indexes audiobooks, interactive simulations, and podcasts.
You can also use the George Mason OER Metafinder to search in real-time.
American Institute of Mathematics
DOAB: Directory of Open Access Books
Digital Public Library of America
CORA: Community of Online Research Assignments
Galileo Open Learning Materials