The Dartmouth Dante Project contains searchable full-text versions of Dante's Commedia and of close to 50 commentaries on it from the 14th century to the present. A few files are in English; most are in Italian. Click on "Help" for general information, including search options, search examples, stop words, and display formats and sorts.
http://dante.dartmouth.edu/search.php
This permalink is a direct, persistent URL to the database that will always work and won't break.