Search for historical plays, melodramas, political satires, black minstrel shows, comic operas, musical extravaganzas, parlor entertainments in the United States from 1820-1900.
Search facsimiles of 15,000 broadsides printed between 1820 and 1900 and 15,000 pieces of ephemera printed between 1749 and 1900 from the American Antiquarian Society.
The Rubenstein Rare Book and Manuscript Library will have many items related to the study of theater.
A good place to start searching their collections is the Collection Guides. You can browse by subject and look for theater to see theater-related topics.