If you're looking for musical scores, try Google Books. Their collection isn't very big just yet, but more are added all the time. Searching can be tricky, so try searching for a unique phrase in the libretto or combine the title of a work with the composer and/or librettist.
Here is a partial list of Opera Scores found in Google Books: http://artsongcentral.com/2008/operatic-vocal-scores-on-google-books/
A list of places where you can find and download free sheet music.