Finding OER
There are a large number of repositories that you can use to search for Open Educational Resources. These two search tools allow you to search across a a wide number of these repositories.
OASIS
SUNY (State Universities of New York) has developed a search tool that currently searches content from 97 different sources and contains over 385,000 records (as of April 2020).
OER Metafinder
George Mason University Libraries, in conjunction with Deep Web Technologies, has created an OER Metafinder that simultaneously searches across 22 open repositories.