When searching in the Library Catalog do a keyword Boolean search and add the word “source” to your search. Example: “Thomas Jefferson” and sources
Other keywords to try:
Library of American Civilization (LAC):
LAC is a collection of materials on microfiche relating to all aspects of American life. Included in this collection are biographies, fiction, letters, texts of public addresses, humor, songs, essays, guide books and count less other topics regarding life in America from the colonial era to World War I. Search for LAC documents in the library catalog and go to the Information Desk for help finding and viewing microfiche.
Some of the books in LAC can be read online (through Google Books & Project Gutenberg.) Quinnipac University has a nice list of what's available online, archived here, and listed alphabetically by author: http://www.terryballard.org/lac.html
Use KU WorldCat and limit to archival sources
The library has a subject page with all its databases that contain primary source documents.