Worldcat.org
This database covers virtually all libraries and archives in the United States & Canada. Like Orbis, it only searches brief descriptions of archival holdings. Select "Format" = "Archival Material" to limit your search to archival sources.
Archivegrid.org
This database is like the Yale Finding Aid database but covers several thousand archival repositories across the United States & Canada. It searches through the detailed descriptions of larger archival collections.