Adding the keywords "public domain" to a search will often return results that are in the public domain when searching in a specific website. For example, in YouTube, searching with a keyword plus the words "public domain" will result in videos that are licensed for the public domain.
The Internet Archive is one large repository of public domain items that spans across material type. Limit your search to a particular media type.
Search for US government websites by limiting your search using the following shortcut when searching Google.
site:.gov + keywords
(site colon dot domain name, works with any such as .ord, .edu, .mil, etc.)
You can also limit to a site or domain such as .org or .edu. Most government documents are free to reuse, redistribute, and remix as works of the US government are published in the public domain. The US government has many published reports and statistics that can be useful pieces of information for any discipline.
Public Domain Content on Social Media
Many US government agencies share copyright free photos through social media accounts. The Creative Commons search box above will search Flickr as one of the options, but you might stay up to date with postings if you follow your favorite US government profiles on your own preferred social media account.
A public domain photo from NASA's Instagram.
You can search the Internet for Creative Commons licensed materials or public domain items. A Google Advanced search allows you to limit your results to items with an open license.