A lot of primary sources are published in books and you can use library catalogs like GEORGE to locate them.
The Library of Congress tags every book that is published in the US with subject headings. There are several subject headings that are used for primary source materials, such as
A more complete explanation is listed here. When you do a search in GEORGE (or another library catalog like WorldCat), try using one of these terms as a SUBJECT search and then adding in a keyword. Your keywords might have to be pretty broad, as you are only searching a very small amount of information about the books - NOT the full text of them.
If you find a book that is relevant for your topic, make sure you look at the subject headings by clicking on the full record tab in GEORGE. This will give you other good search terms. The Library of Congress often uses strange vocabulary and you'll get better results if you can use it effectively.