Hoping this is not a double post - first reply seemed to disappear.
My husband is mechanically restoring a '31 Regular. YT does not have parts for Regulars... so we have purchased parts from the following sources:
Gordon Rice Equip. of PA
Alderson Tractor of IA
Biewer's Tractor Salvage of MN
Have seen numerous clutch plates on Ebay (though I am unsure if they are the type you refer to)... we bought a lot of stuff there: extra manifold, E4A magneto, brass carburetor, potato muffler, small gas starter-tank, brake shoes & springs, user manual, parts manual and the Todd Markle book on F-Series... and some other stuff I can't think of right now.