Loomis House Press have published four volumes of the set, with the fifth in preparation. http://search.store.yahoo.net/cgi-bin/nsearch?catalog=doverpublications&query=ballads

Also, Dover Publications has all five volumes in catalogue at around $25-00 per volume.

While you're checking those two don't forget the Loomis House/Camsco republication of Bertrand Bronson's Traditional Tunes of the Child Ballads. At just $39.95 per volume, that surely has to be the scoop of the century.