Here are some tips for finding materials that are in the Public Domain, as well as those that are licensed under Creative Commons.
To find materials that are not restricted by copyright limitations in Google, go to Google Advanced and in the Usage Rights, choose Free to use or share.
To find openly licensed videos, go to search.creativecommons.org.
Open Educational Resources (OER) are course materials (textbooks, assessments, other instructional materials) that have been created with the express purpose of sharing with others for reuse and remixing. There are hundreds of OER repositories. These Subject Guides can be a starting point:
OER: An Introduction (http://subjectguides.library.skagit.edu/oer-introduction) and
OER: guide to Resources (http://subjectguides.library.skagit.edu/oer-resources)