One's Too Many wrote:
If your plane is a certificated, production model, and not an amatuer-
built experimental, then only an avionics shop that is an FAA-
certificated repair station can legally repair or replace a
transponder system, according to my local FSDO (Houston, TX).

Thanks, that simplifies things greatly.

Calling ATC and
obtaining advance permission to operate transponder-less for the
flight to the avionics shop airport was no hassle at all.

I hope so, since you can't get an inoperative
transponder there any other way.