You can search the online catalog (Primo Pathfinder) for books that contain primary sources. Some subject terms to use are:
The Salmon Library also has the Library of American Civilization (LAC) series. This is a collection of materials on microfiche relating to all aspects of American life and literature from their beginnings to the outbreak of World War I. The collection includes pamphlets, periodicals, documents, biographies and autobiographies, fictional works, poetry, collections of various kinds, materials of foreign origin relating to America, and many rare books not generally available.
You can search for these items by using the phrase "Library of American Civilization". For assistance retrieving specific microfiche cards, ask at the library's Research Help Desk.
Many of these titles have been digitized by libraries, the Google Book scanning project, and the Microsoft Book scanning project.
Once you find your titles check these websites to see if the book is available as a PDF file.
Another collection of primary source documents is found in the series The Library of Original Sources edited by Oliver J. Thatcher.
From the preface: "It is the purpose of this work to present the ideas that have influenced civilization in the words of the men or the documents that developed them."
All the volumes are available as PDF files at the Internet Archive.
Also search for your topic and put the term SOURCES in the subject field. This will bring up documents, letters, diaries, etc.