Sheet Music of the Musical Theater is a collection of more than 16,000 pieces of sheet music published between 1880 and 1922. These songs, specified as “M1508” items in the Library’s classification system, were taken from musicals, revues and operettas primarily of the American and British stage. They are arranged for piano and voice, sometimes simultaneously in arrangements for lower and higher voice.
“It’s Showtime! Sheet Music from Stage and Screen” is a database encompassing these pieces and much more, and can be found here: https://memory.loc.gov/diglib/ihas/html/songsinshows/songsinshows-home.html