Check the Cumulative Index of Volume 31 for Author/Title references from Gale’s Drama for Students. This source is great for summary, context, themes and criticism. If your playwright or play is profiled, there are also individually named authors who have written criticism and these can be cited as a source.
http://tinygaleurl.com?2ygl4qv – Volume 31, Click Cumulative Author/Title index and then do a search for your playwright in the Search Publication box.
Gale products have a nice Google Docs and EasyBib integration. You can save the documents by clicking Download, and the Citation by clicking Cite with Easybib.