Have you tried the Folk Song Society of Greater Boston? Suzanne Mrozak used to (and still may) oversee their library.

Some other places to check would be Warren Wilson College in Swannanoa, NC, Oberlin College Music School and University of Pennsylvania. I assumed you've tried the Library of Congress American Folklife Center.

If these don't work, try contacting Joe Hickerson. He might know of others who would be interested.