A listing of over 5000 websites describing holdings of manuscripts, archives, rare books, historical photographs, and other primary sources for the research scholar.
Browsing through the links, I came across some familiar archives websites, and know from experience that these contain a variety of online resources. For some, you will only be able to access the resources by physically visiting the archives, or contacting an on-site archivist or paid researcher to assist you. These will normally have contact information available, or perhaps a list of local professional researchers for hire. Other repository websites may contain searchable and/or browseable catalogs. Some may even have databases or even scanned photographic or document collections available online!
It behooves you to investigate the Repositories of Primary Sources list...what discoveries you might make!