Public Domain works are not protected by copyright. These could be works not eligible for copyright protection, not creative enough, not created by a person or other factors. Works created by U.S. Federal employees during the course of their duties are not eligible for copyright protection. (Simpson 35) Educational publishers may use many public domain materials because there is no cost to use them, and the publishers can edit the materials as they choose.
Here are a few locations for Public Domain Materials but you will need to double check to make sure their information is correct.