To find primary sources specifically, couple the topic of your subject heading with relevant subheadings that relate to the historical time period, place of origin, and item format of the materials that interest you. Use the Library Search for materials.
These "complex subjects" (containing more than one idea) have their own particular notation.
Claremont Colleges Library subscribes to a database, Proquest Congressional (formerly LexisNexis Congressional) that provides indexing and full-text access to most Congressional publications from 1769 to 2004. Documents and electronic publications after 2004 are generally available through Library Search, the online catalog as well as from FDsys (GPO) and U.S. Congress Web sites.