There are a lot of web sources that are considered legitimate & citable. These are sources found at libraries, theatre companies, and professional organizations that are providing access to materials that they own.
Examples of Theatre Companies' resources
Libraries and Archives provide remarkable resources - examples of such things are
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/
Interpreting Tennessee Williams from the American Theatre Wing http://americantheatrewing.org/videos/kathleen-turner-cat-on-a-hot-tin-roof/
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://guides.library.uwm.edu/go.php?c=2009776
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://collections.lib.uwm.edu/cdm/
The American Theatre Wing Is dedicated to preserving the pat, celebrating the present, and fostering the future of American theatre nationwide (they run the Tony's). Lots of videos of interviews and such.
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.