At work we had good luck with a local upholstery shop rebuilding our forklift seats when they deteriorated like that. Not sure what's around you, but we're only a few miles from Manheim Auto Auction and there are a bunch of recon facilities around.
I have a B7100DT that I bought new in 1978. The seat was torn like yours, went online and found an identical seat. I'm not sure where I bought it, but I think it was on ebay.
I would NEVER buy any Kubota seat even though I live and breathe Kubota simply because they are overpriced. You can buy a better seat from Rural King, Michigan Seat or other online retailers for much less. and seats today are all made to ISO standards with means the will all interchange as far as bolt up is concerned. That especially applies to the sweaty butt vinyl seats. Even my open station M9 that came with a vinyl seat now sports a Cordura seat for literally hundreds less that the Kubota edition.