Check Northwestern Libraries Research Guides for suggestions on digital collections related to different subject areas. Below are just a few examples of guides that have suggestions for digital and online primary source collections. As mentioned before, you can reach out to librarians for suggestions or for help tracking down materials at, and outside of Northwestern.
Just as you would do with trying to identify other sources, consult bibliographies in books, articles, dissertations, and other secondary sources.
Some entries in Oxford Bibliographies list primary sources that could lead you to digital collections.
Normally, you might travel to libraries or archives around the world to view archival collections in-house. The databases below can help search across different archival finding aids to help you identify collections. Once you know a collection exists and where it is located, you can check to see if any or all of it has been digitized.