Are you aware that there are a number of online libraries for sheet music in the public domain? While not an authoritative resource, these sites can be very useful for research, educational exercises and, in some cases, even performance.
The Choral Public Domain Library (CPDL) is a large archive of free choral music. Anyone may contribute, so selections range from early music to the unpublished work of current composers.
International Music Score Library Project (IMSLP) is a WIKI site offering scanned-in public domain scores which can be browsed by composer, time period or instrument.
Werner Icking Music Archive (WIMA) contains an EXTENSIVE list of links to other online public domain libraries.
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