A lot of primary sources are published in books and you can use library catalogs like HoyaSearch to locate them.
The Library of Congress tags every book that is published in the US with subject headings. There are several official subject headings that are used for primary source materials:
When you do a search in HoyaSearch (or another library catalog like WorldCat), try using one of these terms as a SUBJECT search and then adding in a keyword or two. Your keywords might have to be pretty broad, as you are searching only a very small amount of information about the books (e.g., book titles, subject headings, chapter titles)--NOT the full text of them.
For example, if you were looking for primary sources about the Titanic, you could try "Titanic" as a KEYWORD with "sources" as a SUBJECT.
If you find a book that is relevant for your topic, make sure you look at the subject headings by scrolling down to Details in HoyaSearch. This will often give you other good search terms. The Library of Congress often uses a strange vocabulary, but you'll get better results if you can use its subject headings effectively.