Googling the Z500 Kubota block I get:
'This is a marinized Kubota Z500 block, which is the engine used in the Kubota B5100
The tractor model may
help you get through some of the Kubota engine parts online website, or not.
I did not see a thermostat listed for your Z500 block/B5100 tractor at two Kubota places I checked. I'm going to guess that it is a standard thermostat and that if you just take it to a good auto store you will be able to match it by eye and temp. setting.
Have you temperature tested the old one in a pot of water while heating? It should open up if it is still working. Thermostats are not as cheap as a set of spark plugs or wires I'll point out. They are a combination of engineering in mechanics and metallurgy in a fairy sophisticated small metal contraption. For comparison: for my Atomic 4 engine the OEM 140F Tstat is ~ $160, the aftermarket Tstat ~ $85 and the cheaper single action Tstat is $30 to replace. I have the el cheapo model on my engine.
Prices are from here: Moyer Marine Atomic 4 Engine Rebuilding and Parts
Perhaps you have the same Tstat housing?
I recently walked into an auto parts store in Marathon, FL that catered to boaters. I found an aisle that had a good portion devoted to just thermostats, carded in plastic bags for outboards and inboards. I'm pretty sure I saw the identical Tstat as is in my engine but did not have mine with me, so I couldn't be sure and did not buy it.
A lot of words to say that Tstats can be expensive and you should be able to find one, somewhere.