Scholars and publishers compile collections of primary sources that can be found in library catalogs, such as Discover or World Cat. Examples include published manuscripts, diaries, or letters of an individual (In Lincoln's Hand: His Original Manuscripts), or sources related to a particular topic (The Columbia Documentary History of the Asian-American Experience)
Like every other item in the catalog, these collections are assigned standardized subject headings or subheadings, making them easier to find.
To locate primary sources in a library catalog, it is best to use the "Advanced Search" feature, with keywords for your topic in the first search box, and the subject headings for the kind of source you are looking for in the second search box (make sure the two boxes are connected with the "and" dropdown, so you are searching for both concepts).
Example:
Common subject headings for primary sources include:
For a more exhaustive list, with examples, see MIT Libraries' page on primary source subject headings.