Open Educational Resources (OER) are materials designed for student learning that are freely available for use in your course, typically without any payment. Attribution is still the expectation, but these resources can be extremely helpful in delivering course content in an economical manner.
The Mason OER Metafinder (MOM) is a great place to start looking for material with a very broad search across many different OER sites. This metafinder searches many sites simultaneously, rather than a single collection of material. You may find that this is a good place to start your search, or to determine which sites hold the best information for your subject material.