Check Hastings for pony rings, I think I remember them having complete sets for around $35 or so.
I get a lot of parts from Florin Tractor in California, both genuine and AM, in fact I'm just about to order a heap more today, including the bearings and seals for my final drives on my D2 4U, their prices are very reasonable and service is great, talk to Dana or Judy, or email them as I do. I've found if you ask around a few places you can save yourself thousands on a rebuild, I saved almost a thousand just on the D2 final drive bellow seals.
Mike