When something is in the public domain, it's copyright is either expired or it doesn't have one of the conditions of copyright to qualify for protection (creativity, originality, and fixed in a tangible medium). Individuals can also dedicate their copyrighted works to the public domain by applying a CC0 "license" to the work.
It's not as likely that you'll be able to find educational materials in the public domain, unless the course has a focus on history or archival works, but if you think there might be something, you can check out these resources: