Opera and musical theater librettos can be found under the call numbers ML49 or ML50 and under the subject heading of Operas - Librettos. When looking for a specific libretto, try searching for the opera title followed by the word, "libretto." Also, check the liner notes to recordings; the complete libretto and sometimes a translation can often be found here.
Some general sources are listed below.
Some of these resources are located in the Music Library's reference section, and can only be used within the library. Click on the title to check the location and availability.
For prolific opera composers such as Puccini and Verdi, there are many more collections than those listed here. Try an author search on the composer's name (last name, first), then jump to the title, "Operas - librettos," in the list of results.
Many more resources for librettos are available on the Internet.