Open Education Resources encompasses a wide variety of resources, and like with textbooks there are a variety of sites to search for resources. Some sites offer a variety of resources on a variety of topics, some sites are just textbooks, and some are subject specific. You may need to look at a couple of different sites to find what you are looking for.
OASIS was created by the Milne Library at SUNY Geneseo with the goal of facilitating the search for open learning materials. OASIS contains more than 155,000+ records from 52 different sources that have been carefully selected in collaboration with SUNY OER Services. You can limit search results by license, type of resource, subject, source, and reviews.
The Mason OER Metafinder was developed at George Mason University and searches more than 20 OER collections. You can limit your search to OER-specific sites, such as OER Commons or BCcampus, or expand your search to include open content sites like American Memory and Project Gutenberg.
MERLOT - "The MERLOT collection consists of tens of thousands of discipline-specific learning materials, learning exercises, and Content Builder webpages, together with associated comments, and bookmark collections, all intended to enhance the teaching experience of using a learning material. All of these items have been contributed by the MERLOT member community, who have either authored the materials themselves, or who have discovered the materials, found them useful, and wished to share their enthusiasm for the materials with others in the teaching and learning community."
OER Commons is an open repository that contains materials for both K-12 and post-secondary classrooms. Check out "Collections," under the Discover tab, to browse OERs curated for different disciplines and audiences.