ArchiveGrid includes over four million records describing archival materials, bringing together information about historical documents, personal papers, family histories, and more. With over 1,000 different archival institutions represented, ArchiveGrid helps researchers looking for primary source materials held in archives, libraries, museums and historical societies.
A listing of over 5000 websites describing holdings of manuscripts, archives, rare books, historical photographs, and other primary sources for the research scholar.
https://www.lib.uidaho.edu/special-collections/t-abraham/Other.Repositories.html
The Library of Congress provides a Web interface for searching archival and manuscript cataloging in OCLC WorldCat. The Library of Congress only provides search access to the following materials in OCLC WorldCat: archival and manuscript collections; mixed materials (i.e., collections that contain two or more different types of bibliographic materials); and bibliographic materials that have been described using archival descriptive rules.
http://www.loc.gov/coll/nucmc/oclcsearch.html to search the catalog
OCLC Worldcat is also available through CSU-Pueblo, but if you use NUCMC's portal, your search is automatically limited to primary source materials.