Should not be hard to source a rim/rims for your machine, I believe they are the same Bolt pattern from the B5100 up to the B7200, I sourced a set of rims for my B7500 from Agrisupply, They are farm implement rims and I was able to find the correct rim size and bolt pattern. These smaller tractors are not that heavy and farm implement rims are strong enough to handle the weight and stress these small tractors produce
Agri Supply® offers quality tractor wheel rims, agricultural wheels in a range of sizes, and all the hardware you'll need.
www.agrisupply.com
Kubota B6200 Tire specs
AG Front 4.4-10
AG Rear 7.00-16
Lawn/Turf Front 20.5x8.00-10
Lawn Turf Rear 29-12.00-15