Some of the Library of Congress subheadings that point to primary sources include:
Search by Keyword in the library catalog using various terms for your topic paired with a term listed above.
As stated by Jenny Presnell, the author of The Information Literate Historian, "historical research involves building on others' research as well as primary evidence, but, most importantly, formulating a unique research question and developing an argument." (p. 9)
See other guides (e.g. European history) for additional sources.