Servo reversers used to be common, but apparently are now nearly extinct with most all transmitters offering servo reverse and many with model memories. The only one I can find is this one:
SR10 Servo Reverser
Hitec digital servos can be reversed with one of their programmers.
There are a couple other methods that require rewiring surgery, either the transmitter (not practical for your situation) or the servo. Challenge with the servo is most are now surface mount electronics, might be more difficult to rewire than old discrete component stuff.