If you are doing a keyword search in a library catalog and want to find primary sources, try using terms like "sources," "diaries," or "correspondence." For example you could type in: "American Revolution" and "sources."
You can also search by author if it is a notable individual like George Washington. Under author, type in Washington, George.
You can also search for what is called a corporate author, which means the author is an organization, not an individual. For instance, if you want to find documents produced by the Parliament in the UK you could look for the author Great Britain. Parliament