Search the Randall Library catalog (and other libraries) for history makers' names as authors. This should retrieve their memoirs, diaries and correspondence.
As you search subject topics in the catalog, note that the following subdivisions indicate primary source (* indicates it can be used with individuals' names,classes of persons [e.g., soldiers], and organizations) :
More suggestions can be found on the "Primary Sources" guide.
Here are just a few useful free sources for U.S. Civil War primary sources: