A great place to start your search for OER is an OER search tool, such as OASIS or the OER Metafinder. These tools allow you to search some of the main trusted sources of OER through their interfaces.
*Search OASIS 
Openly Available Sources Integrated Search (OASIS) is a search tool that aims to make the discovery of open content easier. OASIS currently searches open content from 115 different sources.
This is a search engine for open access textbooks and OER from George Mason University. It searches across 22 different sources of open educational materials:
Mason OER Metafinder (MOM)
You can also search individual OER and open textbook collections, listed below, or find open access or licensed content through the WPI Library Search.
When in doubt, ask your librarian! We can help you find OER to use in your courses.
The B.C. Open Textbook Collection is home to a growing selection of open textbooks for a variety of subjects and specialties. Discover open textbooks that have been reviewed by faculty, meet accessibility requirements, and/or include ancillary materials (quizzes, test banks, slides, videos, etc.).
GALILEO Open Learning Materials brings together open educational resources from throughout the University System of Georgia, including open textbooks and ancillary materials.
14 libraries or collections of Textbooks, Textmaps, and LibreTexts
Milne Open Textbooks (formerly OpenSUNY textbooks) publishes textbooks authored by SUNY faculty in a variety of areas.
OAPEN works with publishers to build a quality controlled collection of open access books and provides services for publishers, libraries, and research funders in the areas of dissemination, quality assurance, and digital preservation.
Free, adaptable, openly licensed textbooks and supplemental resources from OER Commons - organized by discipline and provider
A leading independent Open Access publisher in the Humanities and Social Sciences in the UK: All books are available to read online and download for free, including monographs and textbooks in all areas.
List of 200 free textbooks curated by the Open Culture website. No searching capabilities beyond doing a Ctl-F/Cmd-F search in your browser window.
The Open Oregon State initiative publishes textbooks authored by Oregon State faculty; also available, a list of textbooks in production.
One of the leading open textbook publishers, based at Rice University, OpenStax publishes open textbooks mainly in science, math, and social science fields.
Collection of open textbooks, many of which have been peer-reviewed, maintained and hosted by the Center for Open Education and the Open Textbook Network.
This directory provides an index of 3,374 openly licensed books published across 115 Pressbooks networks.
Open textbooks are available to use, keep, revise, and share under open licenses.
The University of Minnesota publishes open textbooks authored by their faculty, mainly in the areas of business and humanities/social sciences.