Library of Congress subject headings are a good way to uncover scholarly books that discuss the historical and cultural context surrounding the play you are researching. They can also help you find primary source materials (such as paintings and photographs) that may inspire your production choices.
In addition to using Library of Congress subject headings like those above, it pays to spend time exploring the library catalog and brainstorming different combinations of search keywords. For instance, "Expressionism drama United States" pulls up a more focused set of results in discoverE compared the general heading of "Expressionism (Art)." They are both potentially useful, but they serve different purposes.
Documents, photographs and reproductions of artworks can provide a tactile connection with the world you are researching, and they can serve as a source of creative inspiration for your design choices. These are only a few options that you can explore online.