T50 gearbox - TRD 2nd and 3rd gear

[Project D]

The TRD set A (A851) as been discontinued for years and the chance to find a used set in good condition is pretty low.
(plus it did not solve the huge gap between the 3 and 4, so doesn't worth it IMO)

For the KDX it's even worse, discontinued for at least a decade (the company doesn't even exist anymore)

Revolver; as far as I know they are still in production so you can buy them new.
Ask shops like RHD, nengun, Perfectrun etc.. if they can get it for you.

TTS have a set available
MHM too.