Do a search in Google or one of the databases listed on the primary sources tab.
Combine keywords that describe your topic with keywords to retrieve primary sources or digital collections from archives and libraries.
Common primary source keywords include:
You can also try keywords for kinds of primary sources:
Remember to read laterally -- who digitized the image? Was it produced by an archives, museum, governmental agency, or educational institution? The best sites for primary sources will provide you with the information you need to cite your sources.
To see if a librarian has already identified digital primary sources related to your topic, do an Internet search for a term or phrase related to your research plus the phrase "research guide" or LibGuide.
For example: Search for history of medicine "research guide".