You can try Steel Wheel Ranch, they have very good prices for used parts. They also sell some new parts as well for the Farmalls. Mike has always treated me well and provided very good parts, and is fast shipping out stuff.
His address is:
Steel Wheel Ranch
1215 Plumtree Rd.
Everest, KS 66424
They do have a web site also, but I don't think that all used parts are listed.