Works in the public domain (PD) are not protected by copyright. Attribution is not a legal requirement, but scholars traditionally cite PD works to avoid plagiarism.
There are thousands of places to locate PD content. Since much of the content ends up in the PD due to expired copyright, archives are generally a good place to look. Also, try your library catalog, or locate newer content through websites with U.S. government publications or websites that let you filter to CC0 or public domain content. Here's a list to get you started. Be sure to check each item for copyright restrictions as most websites contain a mix of copyrighted content alongside PD or CC works.
My top three pics:
...and many more sites:
This work contains links to content in the public domain and content with a variety of licenses or copyright restrictions. Additionally, the uploaded items under examples are in the public domain.