It depends on which song you are asking for. If it is a P.D. American popular song, Levy, American Memory, and other sheet music collection sites would be helpful (Click here for the links). It is much harder to find a score for a copyrighted song on the web, as well as a song in fieldwork collections. Often, easiest way may be to find a songbook containing the song, using such title indexes as Plymouth Song Index.

~Masato