Try April 1930s
http://www.april1930s.com/html/singe...bobbins__.html
While she no longer carries the replica Featherweight bobbin cases, you can send her an Email to see if she has a an original from a parted out machine. Her's originals are pricey though at $89.95.
Sew Classic
http://shop.sew-classic.com/Bobbin-C...-SCBC45751.htm carries the replicas and each and everyone that she sells has been tested in a sewing machine and is guaranteed to work. Her price for the replica is $39.95.
I have done business with both sellers.