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.
Your College Library has eBook databases that may contain your current textbook! Or you may be able to find an eBook in it that you can use instead of a paid-textbook. Or you can ask your librarian to see if there are any eBooks that can be purchased which will cover the same content your current textbook covers. Students can access this quality content for FREE. The eBook needs to have a special user permission called unlimited use. This means that all students can access and use the eBook at the same time. The user permissions are listed in the details of the eBook.