OER
You can search for OERs with Google by using the advanced search options and filtering items listed as "free to share, use, or modify" in "usage rights".
You can use the Creative Commons search engine to find images that you can use and remix across several open archives.
You can create OERs (textbooks, worksheets, videos, tests) and share them with an open license on the CUNY Academic Works' Open Educational Resources website.