Archives and special collections are by their nature difficult to identify with a central database. Small special collections are frequently missed and large collections have material that haven't even been described yet.
It is important to go beyond these indexes and contact libraries and archives that might have relevant resources to speak with the local experts.
Special collections outside of the United States and England often lack central repositories of information, so one often needs to approach the collections directly. A lists of many archives in countries around the world are available here and here.
Provides centralized access for searching and retrieval of archival records and finding aids, including finding aids indexed centrally at RLG and others located on servers around the world and records in RLG's AMC (Archival and Mixed Collections) file. AMC provides researchers with timely and comprehensive access to brief, high-level information for nearly half a million collections of manuscripts and archives.
Database containing the records of all material cataloged by OCLC libraries, including hundreds in the United States and some major libraries in other countries. Manuscript collections that are cataloged by these libraries will appear in Worldcat.
A2A allows you to search and browse for information about collections of records (archives) in England and Wales, dating from the eighth century to the present day. The archives described on A2A are cared for in local record offices and libraries, universities, museums and national and specialist institutions across England and Wales, where they are made available to the public.
A2A contains about 30 per cent of catalogues of archival collections in England and Wales. The database now contains 10.3 million records relating to 9.45 million items held in 418 record offices and other repositories
The NRA is a central point for the collection and dissemination of information about the nature and location of manuscripts relating to British history. It currently consists of over 44,000 unpublished lists and catalogues that describe archival holdings in the United Kingdom and overseas
AIM25 is a permanent web-accessible database of descriptions of the archives and manuscript collections of more than one hundred of London's higher education institutions, learned societies, cultural organisations and City livery companies. The website has been fully available since 2002. New partner institutions and records are added regularly with the objective of London-wide coverage of all the capital's archive holdings.