A lot of primary sources are published in books and you can use library catalogs like the Get Started Page to locate them. There are a couple of tricks to doing this.
The Library of Congress tags every book that is published in the US with subject headings and there are several subject headings that are used for primary source materials, such as
When you do a search in the Discovery, 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 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 that describe the book. Often this will give you other good search terms. The Library of Congress often uses strange vocabulary (Indians of North America is how they tag Native Americans) and you'll get better results if you can use it effectively.
Another helpful place to find other sources is in the bibliography at the end of a monograph, or the references in an article or encyclopedia article.