Web Sources from professional theatres can be incredibly useful, and are considered legitimate & citable.
The National Theatre - in London has a large collection of resources for learning about working in the theatre
Libraries, Professional Associations, and Archives provide remarkable resources as well - they often have the papers, photographs, diaries, etc. of famous actors, playwrites, directors, as well as the archives of actual theatres and theatre companies.
The New York Public Library http://www.nypl.org/search/apachesolr_search/tennessee%20williams
The Tennessee Williams / New Orleans Literary Festival http://www.tennesseewilliams.net/
The papers of Tennessee Williams are at Columbia University http://www.columbia.edu/cu/lweb/archival/collections/ldpd_4079626/index.html
The UWM Libraries has the photo archive from the Milwaukee Repertory Theatre. You can see how local productions of Tennessee Williams' plays were presented http://uwm.edu/lib-collections/mkerep/
Other digital collections at the UWM Libraries might help you put plays in context. For example political upheaval that affects how a playwrite worked might be reflected in their setting for the play and photographs of a city in that time might give you an idea of why it was chosen. The UWM Digital Collections pages are all available from here http://www4.uwm.edu/libraries/digilib/
The Margaret Herrick Library of the Academic of Motion Pictures Arts & Sciences (the Oscars folks!) has an amazing array of materials on early film. Scanned journals, posters, even digitized film.
American Theatre Wing - the home of the Tony's, runs a series of video and audio interviews on a regular basis. With actors, directors, critics, anyone involved in theatre production. Everything (or nearly so) is archived online. A few examples: