Madness in Hamlet
Shakespearean Criticism contains several relevant articles discussing madness in Hamlet.
For a particular theme in a Shakespeare play, the Shakespearean Criticism reference set has a "Cumulative Topic Index, by Play." It is organized by play and underneath each play, gives various topics with the volumes and page #'s that refer to that play and that topic. When doing research on themes in Shakespearean Criticism, you may need to look under different headings.
Shakespearean Criticism also has a "Cumulative Character Index" and "Cumulative Topic Index" organized by topic and then broken down by play. These indexes are at the back of every volume.
Library databases are also helpful for finding information on a particular theme in a Shakespeare play. For information specifically on the theme of "madness," go to the library homepage; select the link for Literary Criticism under Topic Databases on the right-hand column. Start with Gale Literature Resource Center and key in "Hamlet and Shakespeare and madness." This search lists 4,000+ articles of literary criticism.