Most popular search tools:
Sources for Sheet Music:
Lester S. Levy Sheet Music Collection via Johns Hopkins University
Sheet Music Consortium -- "provides tools and services that promote access to and use of online sheet music collections by scholars, students, and the general public."
Online Digital Muscial Documents --"a list of online digital musical document libraries. Each source listed below offers access to collections of digitized music documents (typically originating from printed or manuscript musical sources), and containing music notation of some kind, stored as an image file."