I think there's no 'official' way to get one, you can make your own starting from the schematics that you can find in the thread. The difficult part is to program the PIC (or other microcontroller used) with the correct program. I think that through the posts you get enough info to write one, or Howard might be distribuiting his program that work with his decoder (no Mylaps compatibility).
The transponder can be prototyped quite easily as shown in the images, I'm about to test a new 8 pin PIC that should be able to do everything with just 1 chip.