You will get limited results if you try to search for "primary sources" in library databases since the Library of Congress does not use that subject heading. Try searching these subject headings instead:
memoirs or autobiographies
letters or correspondences
SOME ADVICE ON AUTOBIOGRAPHIES: You will get limited results searching for the word "autobiography" since the Library of Congress does not use that subject heading. Try searching for "biography" but then look to make sure that the author is also listed as a subject. That is your clue that the person is writing about him or herself, and that is a primary source.
DOCUMENTARY HISTORIES: There are collections of documents relating to various subjects, such as Agriculture in the United States: A Documentary History. Search IDS for these by typing your subject along with the phrase "documentary history" in quotes.
DOCUMENTARY FILMS: Be wary of "documentaries" (films) because sometimes they are primary sources but often they provide analysis that is secondary.
PUBLICATION DATES: Be aware that publication dates can be misleading. A book could have been written in 1952 but republished with a 20013 publication date; this is still a primary.