The Bad news is that there is not a single source to find them all in one search.
Things to consider:
What are your criteria? (the must haves when searching for physical & digital archival content): geographic, type of content, representation of specific populations, age of content, format of content, other?
Some Finding Aids on finding digital and physical archives include:
The National Archives: Archival Research Catalog
OAIster: a union catalog created from the Open Archives Initiatives Protocol for Metadata Harvesting
and there are many others
Hamline Research Guide: Locating and Accessing Primary Documents