You can only change the frequency within the band it is set in, so only within 40MHz. There were a few frequencies used (IIRC 35, 53 and maybe 60 MHz, was a few years ago). These radios have now all been replaced by the 2,4 GHz radios, where you don't have to worry about interference from other radios. So to find an old transmitter you need to visit hobby shops. It might also be possible to replace the receiver in the car with a 2,4 GHz one and use a new radio.