If you have questions about finding aids or an archives' holdings, look for contact information and contact the archives directly. Archivists know their collections well and can help you learn more about a collection, whether you can order reproductions of materials, etc.
If you are planning to visit an archives, it's best to contact the archives directly before you visit, to learn about registration procedures and other rules the archives may have for using their resources.
WorldCat includes information on archival holdings.
For best results, search by name or other proper noun.
Select "Archival Materials" in the "Format" dropdown menu.