If you can locate an archive or library with original sources that interest you, you may be able to request digitized copies.
These four databases enable you to find specific archival and manuscript collections. They overlap, but each contains unique material.
Archive Finder -- an online database of archival materials in the US, UK, and Ireland
WorldCat -- in the Advanced Search, under "Limit Type To," select Archival Materials. Alternatively, under "Subtype Limits," choose Manuscript as the format.
Archive Grid -- a database containing all the archival and manuscript records in WorldCat (except dissertations), along with thousands of finding aids
Social Networks and Archival Context Project (SNAC)
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Collection -- a gathering of primary materials considered as a unit, e.g. the Black Hero Comics Collection or the John Updike Papers in the Houghton Library
Repository -- the place where an archival collection is held, such as the Houghton Library or Schlesinger Library
Finding Aid -- an outline of the contents of an archival collection
Collection Level Record -- a catalog record for a collection as a whole: e.g., Apollo Theatre News Clippings
Item Level Record -- a catalog record for a single item in a collection