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 is available here.