There is a multitude of OER to choose from, ranging from full textbooks and courses, multimedia resources, and primary sources.
Video: How to Find and Evaluate OER:
Abbey Elder OER Starter Kit
Open Educational Resources are teaching, learning and research materials in any medium - digital or otherwise - that reside in the public domain or have been released under an open license that permits no-cost access, use, adaptation and redistribution by others with no or limited restrictions.
-Hewlett Foundation https://hewlett.org/strategy/open-educational-resources/
Video: CC BY 4.0 by the Council of Chief State School Officers, Music: The Zeppelin by Blue Dot Sessions: CC BY NC 4.0