You can use WorldCat to help locate manuscript collections across the globe. After typing in your keywords, you can LIMIT your results to "Archival Materials" before hitting the search key.
CIS Index to Publications of the United States Congress ((KF 49.C621x, Gov.Documents, Index Tables)
This is a monthly publication that provides detailed indexing and brief abstracts of the following types of Congressional publications: committee hearings, House and Senate documents and reports, and Senate treaty documents. Indexes are arranged by subjects and names (including names of witnesses at hearing, etc.); titles; bill, report and document number; and committee and subcommittee chairs.
National Union Catalog of Manuscript Collections (Z 6620 .U5 N3 Zhang Legacy Collections)
Compiled by the Library of Congress, this multi-volume set includes reproductions of catalog cards for manuscript collections reported by repositories throughout the United States during each period covered.