There are a number of different databases and search tools that will help you locate archival records and manuscript collections.
Tools for locating primary sources in the history of health and medicine
History of Medicine Finding Aids Consortium: union database of finding aids for collections held by health sciences libraries.
National Library of Medicine/History of Medicine: search tools and lists of finding aids for the primary sources held by the National Library of Medicine.
Wellcome Library Archives and Manuscripts: Subject guides and search portal for history of medicine and health archives and manuscripts held by the Wellcome Library.
Other general tools for located archives and manuscripts
ArchiveGrid: Catalog of archives and manuscript collections
Archives Finder: searchable database of collections of primary sources housed in the United States and United Kingdom.
National Archives: online public catalog for the U.S. National Archives.
Virginia Heritage: union database of finding aids from Virginia archival repositories.