Murn Tire & Tool is a good place to look. http://myplace.frontier.com/~mtnt/index.html I'm not sure if RB Leaman in Pennsylvania is still doing NI uni-system parts or not . . . he used to be the go-to guy in this area but I don't have contact info for him anymore.
When I drove past http://bryantcombineparts.com/ the other day, I saw a couple of corn heads that looked like what you were describing. You might give them a try. They also had several complete looking UniSystems sitting on the lot, and several parts machines out in the back.