Resources can be found all around the web, but be careful to check your sources! Other places to look for Primary Sources are:
University/College Theater websites - Look for a link to Past Productions, these may lead you to production pictures, technical notes, programs, posters, etc.
Explore 894,229 items digitized from The New York Public Library's collections. This site is a living database with new materials added every day, featuring prints, photographs, maps, manuscripts, streaming video, and more.