"Primary sources are 'fundamental, authoritative documents relating to a subject, ...e.g., original records, contemporary documents, etc.' (Young, Heartsill, ed. The ALA Glossary of Library and Information Science. Chicago: American Library Association, 1983, p.176). Primary source documents are first-hand accounts by a direct participant or observer and may include letters, diaries, interviews, photographs, films, maps, government documents, and more.
For the arts, history, and humanities, original primary source documents usually are housed in museums, archives, restricted library collections, and government offices. Reproductions of primary source documents often can be found in online digital collections, microform collections, books, and other secondary works."
A lot of primary sources are published in books and you can use the library catalog 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
A more complete explanation from Georgetown University is listed here. When you do a search in the Phillips Library Catalog 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 encylopedia article.