There is no precise way to find collections of digital primary sources. Search tools that you use for searching for primary sources whether they are tangible or digital are the same. You can use catalogs, ONESEARCH & WORLDCAT (a catalog of individual material titles & collections in the world), databases, & search the internet. When looking for digital/online primary sources, some will be free & some only accessible if our library subscribes to the collection.
Also, browsers will find non-digital primary sources. When looking for primary sources, the right primary source for your research is the most valuable, regardless of format. The best source may not exist digitally, so we want to be open to finding the best sources for our research & assignments