From time to time in the past, I have written about the Digital Public Library of America. As you can see from this screenshot, the DPLA now has well over 44 million images, texts, videos, and sounds from across the United States. If you visit the website, you will also see this link:
What’s the deal with copyright and a DPLA item?The copyright status of items in DPLA varies. DPLA links to a wide variety of different materials: many are in the public domain, while others are under rights restrictions but nonetheless publicly viewable. For individual rights information about an item, please check the “Rights” field in the metadata, or follow the link to the digital object on the content provider’s website for more information.
You might want to add a search in the DPLA to your research methodology.