The Society of American Archivists provide some additional suggestions for finding materials in archives.
ArchiveGrid allows you to search more than 1,400 archives, libraries, museums, and historical societies holding millions of online or on-site records. They may be run by government agencies, public or private universities or non-profit organizations. Archives generally offer “finding aids” outlining their holdings. The Society of American Archivists provide some additional suggestions for finding materials in archives.