The library has many primary sources and primary source collections. To find primary sources, try searching for your topic plus possible types of sources, such as "documents", "diary", "speeches", "sermons", "letters", "papers", and more.
For example, you could search for "slaves AND 'revolutionary war' AND papers" (make sure to separate your keywords with operators like AND or OR).
Use Interlibrary Loan to request books or journal articles that the UM Libraries does not have access to.