The following databases are useful for locating archival materials outside of Purdue:
ArchiveGrid: ArchiveGrid contains descriptions of archival holdings from libraries, museums, and archives nationwide.
Worldcat: Worldcat is a network of libraries. Going to "Advanced Search" and limiting by format will point you to archival materials from many different types of repositories around the world.
Other useful places to search for archival materials include local historical societies, state archives, and research libraries. Examples of these types of repositories include: