Primary sources can often be found online either through freely available digitized collections such as the Library of Congress' American Memory Project (see link below) or through commercially produced digitized collections such as HIstorical Newspapers Online.
The U-M Library subscribes to or purchases many commercially available collections which are accessible to currently affiliated U-M faculty, staff and students on campus and remotely. Many of these are also available through selected guest workstations on campus to the visiting scholars and the general public.
Freely available digitized collections can be found through U-M research guides, by searching the web, and by reviewing bibliographies and footnotes for collections relevant to yours interests.
Examples of commercial collections which the U-M Library provides access to are: