I wish you the best of luck trying to find the fronts {4 lug}. It took me over a year to locate them, mfg discontinued. Kubota still makes a 4 lug wheel but its a different size from the older 4 lug B7100's, mine is 1982-86.
How I know this I bought one from a Bota dealer and before I checked the diameter of the wheel compared to mine I painted it to match, My Bad. and I could't expect the dealer to accept a return on it after I painted it.
Anyway I continued searching the WWW and finally located a private sale from a listing that was already over a year old.....took a chance.....contacted the seller. After 2 months the guy finally checked the site he posted on and contacted me. I snapped-em up. All I asked was that he have the old tires removed for shp'g to keep the weight down, and I gave him $5.00 each for each tire to be removed. Plus the asking price.....
And really happy I got em....so the best of luck to you finding them if your B7100 is around the same age as mine.
OH BTW if yours is the newer 4 lug type, I have 1 brand new wheel painted with the earlier Kubota orange color. Never used.