Try calling these three places that deal in a lot of used parts:
Jack First, Inc.
1201 Turbin Drive
Rapid City SD, 57701
Phone: 605-343-8481
Pennsylvania Gun Parts
RD 7 Box 150
Mt. Pleasant PA, 15666
Phone: 724-547-3257
Wisner's Inc.
P.O. Box 58
Adna WA, 98522
Phone: 360-748-4590