To Terry Smythe's list of good sheet music sources on the Internet,
I would add this:

  Johns Hopkins University's Lester S. Levy Sheet Music Collection
  http://levysheetmusic.mse.jhu.edu/ 

As long as the sheet music was published before 1923 the whole of the
sheet music is shown, including front and back covers.  If the music
dates post-1922, only the front cover is shown.

Thank the influence of the Disney people and the late Congressman Sonny
Bono for preventing that cutoff date from advancing one year each
January first.

Matthew Caulfield
Irondequoit, New York