I’m struggling to source a new piston for the rear brake caliper of my 2002 Tmax 500.

It’s no longer manufactured. I have seen that people have, as a direct replacement, fitted a Yamaha R6 early model four pot front caliper to Tmax’s.

Has anyone come up with a reasonable upgrade for the rear?

Is your rear piston beyond a clean up?
If you go to the fowlers parts site and get the part number, piston may be used on other models that may be available as a patern part...
or maybe Wemoto?

Depends on how much you want to spend, as you can fit an adapter bracket plus then the cost of a suitable caliper

https://www.ebay.co.uk/itm/Tmax530-Tmax ... SwCkxc-Hbp