Open Educational Resources are found in numerous...almost infinite...places online! There's no single place that endeavors to collect every OER in existence. So, to make life easier, most OER materials can be found through OER directories which pull content from various online sources. You'll find materials ranging from textbooks to videos to ancillary materials.
If you are interested in exploring your options with traditional OER textbooks, these directories will point you in the right directions. Each one is different in its scope and aim, so play with each of them to see what turns up!
You can check out these much more comprehensive directories for other options...there really are hundreds and hundreds of OER directories online now!