If you want to find primary sources quickly, on a number of important themes and subjects, use the digital archives in the ever growing collection Archives Unbound.
For a more thorough search for primary sources, especially those found in the many archival repositories in the Yale University Library system, consult this website.
Some of the tools one can use to find these sorts of collections include:
- Orbis is the most comprehensive tool for finding primary source collections at Yale; use the "more limits" feature to limit searches to primary sources.
- Yale Finding Aid Database is probably the best tool for finding archival collections at Yale, and more user friendly than Orbis.
- Worldcat is the most complete source for finding collections around the world.
- Archive Grid is also a great tool for finding collections, both at Yale and beyond, though it's not as complete as Worldcat.
- Archive Finder is another tool for finding manuscript collections. The best use of Archive Finder is the repository search, which you can use to find archives in specific geographic locations.
- Connecticut Archives Online is a union catalog of archival collections held in Connecticut repositories.
Keep in mind that a very good - and often overlooked - source of primary source material at Yale can be found in the Microform Reading Room in the basement of Sterling Memorial Library. This link brings you to a list of major microform collections available in the Microform Reading Room.