A nice introduction to the various collections at the Library of Congress dealing with Dance, Theater, and Music. The section on Music covers American Musicals. Click on the Table of Contents for further information.
The PAE provides information about the collections of scores, sheet music, audio recordings, films, photographs, etc. Includes digitized items; special Web presentations on topics and collections; articles and biographical essays; finding aids to collecti
Very interesting -- the Performance Arts Resource Guide on the homepage. If you "Browse by Subject", you might try "American Musical Theatre, Dance, Modern Dance and Theatre among the many links to the Library's wonderful collections.
Guides at other institutions:
NYU Actors' Page
Useful to plan research and access some resources. Many links are restricted to the NYU community.