A good place to start is with the Northwestern University Subject Librarians and the Research Guides for specific departments and areas of study.
To see a list of the digital primary source databases available through NU Libraries, go to the A-Z Databases page and limit the database type to "Digitized Primary Sources." Then filter by your subject of interest in the top left drop-down box.
Some examples:
Wiley Digital Archives
Gale Archives of Sexuality and Gender
Gale Archives of Political Extremism and Radicalism
The Sixties: Primary Documents and Personal Narratives 1960–1974 from Alexander Street