Frank-
12X12 rims are almost impossible to find, I don't know if they were never used in the GT market, or what, but it seems like guys who have either 10" or 12" wide rims for sale want a mint for them.
Something to think about though is if you really want a 12" rim anyway. Sure, the tires are going to have a crown on them with an 8" or even a 10" rim, but I believe with plenty of ballast (read = lots of weight) and the correct air-pressure you can get them to squat down pretty good anyway. Remember something that some guys forget.. A Super GT is going to need more rear weight because the engine is further forward with the longer frame. I run wheel weights that are 100lbs a piece and with my 250lb butt in the seat feel as though my 1872 is just barely weighted properly, although at Travis' I could alternate between spinning the high-side wheel and loosing steering control up front, so maybe I was just right.
Another thought to consider is that unless you're plowing by yourself, you'll most likely be running in somebody else's 10" plow furrow, so do you really want a 12" wide "footprint" anyway?...
I may be all wet here, so...
<FONT SIZE="-2">IMO, FWIW, YMMV, My $0.02, Yada, Yada, Yada...</FONT>