If you are interested in researching collections outside of what ESU's archives and special collections holds, while you can do a Google search to find archival repositories, it is better to start with a more targeted search.
The following resources will help you discover archival collections.
ArchiveGrid: Allows you to find repositories that have archival resources.
Digital Public Library of America(DPLA): This site has millions of materials from libraries, archives, museums, and other cultural institutions across the country available to all in a one-stop discovery experience. There are primary source sets and collections that have been curated on specific topics. You can then visit the archival repositories where these items are held to find more information.
National Archives: Holds records of the federal government.