To find librettos:
First, try searching the title and author of your play or musical in Summon, the library catalog. For musicals, include the word "librettos," and limit your search results to Content Type --> Books, e-Books
If you do not find what you're looking for, your play or musical might be contained in a collection with other works, like in an anthology or periodical. Check for your play title the following indexes and locate the title of the larger anthology or periodical. Then, search for that larger title in Summon.