Since posting this I discovered that Valenti tractor in Troy N.H. is out of business.
Closed down, gone, although there is still a back to front fork lift Branson in the yard, plus a lot of other junk.
For my own joy stick boot - I have been making do with a plastic bag, that won't make it through the winter.
I have cleaned up the splits in the original boot and glued them together, they seem to be holding.
Plan next is to try and "paint" it with rubber cement, initially on the outside and if that makes it strong enough to do so I will turn it inside out and do the other side.
I did look for some generic joystick boots, e.g. at Surplus supply, but nothing seemed close enough.
Hard to believe there isn't a Caterpillar, Case, JD, or whatever else that would fit.
Another search turned up this;
New OEM KIOTI 36��1-�18� Loader Joystick Valve Handle Cover Boot 3SOF2�4111 | eBay
It looks very similar, most seem to be rectangular but this looks as if it is square - of course it could be too small.