As said, zero offical supply or support for anything with a TRF part number.
Tamiya/TRF have decided to completely scale back off-road - none of their international drivers compete in the class anymore and the best of the lot - Lee Martin - was let go shortly after the release of the 503. Development would appear to have stopped and with that, probably production too 503's/201's are thin on the ground.
Wouldn't be shocked if they lose interest in on-road soon too if the Naoto experiment doesn't work - making the B main isn't going to impress anyone.