Opera and musical theater librettos can be found under the call numbers ML48 - ML50 and under the subject heading of Operas - Librettos. When looking for a specific libretto, try searching for the opera title followed by the world, "libretto." Also, check the liner notes to recordings; the complete libretto and sometimes a translation can often be found there.
Some general sources are listed below.
For prolific opera composers such as Puccini and Verdi, there are many more collections that those listed here. Try an author search on the composer's name (last name, first), then jum pto the title, "Operas - librettos," in the list of results.
Below are examples of song translations and transcriptions for specific composers. To browse others, try a subject search on "Songs -- texts."