There are a few good starting places to locate public domain materials. Some are general interest but many are content focused. Knowing your content can help you get to the materials more quickly. 

National Archives Everything produced by the federal government is automatically in the public domain. The archives is a good place to start for finding historical artifacts.)

Library of Congress While not everything at the LOC is in the public domain, much of it is and licenses are usually pretty easy to locate using the Rights and Access link associated with a resource.  Check the screen shot to see the link and the language provided.)

NASA: http://www.nasa.gov

Yellowstone National Park: https://www.nps.gov/features/yell/slidefile/index.htm

New York Public Library: https://www.nypl.org/research/collections/digital-collections/public-domain

Ideas for Classroom Use: https://www.nypl.org/blog/2016/01/20/public-domain-in-the-classroom

Metropolitan Museum of Art: http://www.metmuseum.org/art/collection

More From Wikipedia: https://en.wikipedia.org/wiki/Wikipedia:Public_domain_image_resources

More from The Public Domain Review:

https://publicdomainreview.org/guide-to-finding-interesting-public-domain-works-online/