One key issue here is the distinction between primary and secondary sources. If you are writing about Dario Fo, primary sources would be his works, interviews with him, and his letters or own critical works. Any works written about him rather than by him would be considered secondary works. Similarly, in studying commedia dell'arte, primary works would be somewhat difficult to locate, as the performances were improvised, but one can find plays based on the tradition of commedia and contemporary discussions of the genre.
Many of Dario Fo's works are available in English translation, including A Woman Alone and Other Plays and The Tricks of the Trade, and two collections, Plays One and Plays Two. Another useful primary source would be his autobiographical work,
Fo, Dario. My first seven years (plus a few more). Trans. Joseph Farrell. New York: Thomas Dunne Books, 2006.
Some other useful sources would be:
Farrell, Joseph and Antonio Scuderi, eds. Dario Fo: stage, text, and tradition Carbondale: Southern Illinois University Press, 2000.
Mitchell, Tony. Dario Fo: people's court jester. London: Methuen, 1986.
Scuderi, Antonio. Dario Fo: framing, festival, and the folkloric imagination. Lanham, Md. : Lexington Books, 2011.
For additional scholarly sources, your best places to search would be the MLA International Bibliography and JStor, both of which should be accessible through your university library's website. A full bibliography of Fo's works (through 1989) appears in:
Mitchell, Tony, comp. File on Fo. London: Methuen, 1989.