Don't bother searching. I extensively searched 4 different aftermarket brands (Stant, Gates, Prestone & Motorad) at 6 different auto parts stores and never came up with a match. From there the local Honda cage dealer tried to run some cross-reference numbers to see if the ST13's t-stat was used in any other Honda vehicles. That came up with a big zero, so he let me physically rifle through all of the Honda cage t-stats in his stock (a lot of them) and again I failed to come up with a match.
The Turbo City t-stat is made by Motorad, but Tom had these specially made for the ST1300, by sending them an example of the OEM one. As far as I know you can't get these anywhere else.
What's your objection to using the Turbo City one?