If you are specifically looking for textbooks, start with the Open Textbook Library and the links under OER Producers. For ancillary materials, including assignments, videos, tutorials and the like, start with MERLOT, OER Commons and other OER repositories. The resources under OER Searching Tools are useful for searching across several sites and finding things that might otherwise be overlooked. OASIS and the Mason OER Metafinder are particularly useful for this purpose.
Be aware that some sites, like MERLOT, accept materials that may be published under conventional copyright or low-cost rather than free to use. Always check for copyright or CC license for any materials that you want to use.
You can now find books from the Open Textbook Library when searching the Upstate Library catalog (discovery search)! Simply include "open textbook library" in your search. You can also limit results to "open access" after a search to find openly-licensed materials.
Your subject librarian can help you to find OER and also recommend appropriate library subscription materials.