Kubota is really the only source unless you have a set of rims to use(then you could just buy the appropriate tires.) They are spendy, but worth it IMO. I just put a full set of R1 tires on my B3200, purchased them through whole goods. They were $1080, but yours should be a good bit less since they are smaller.
Thanks Verticaltrx but I really like the turf tires 95% of the time I use my tractor. This has been a long running debate between lovers of different tires. The nuisance of changing rubber would be too much.