Going directly to publisher websites to search for music can be a very handy tool, especially if you know already what publishers will have works of particular composers.
Here are a few suggested websites for you to search works of particular composers
Boosey & Hawkes: http://www.boosey.com/
Theodore Front Musical Literature: http://www.tfront.com/default.aspx
J.W. Pepper Sheet Music: http://www.jwpepper.com/sheet-music/welcome.jsp
Trevco-Varner Music (for woodwinds): http://www.trevcomusic.com/
Shar Music (for strings): http://www.sharmusic.com/