For my DC-30 Garmin unit, I had trouble finding one until I realized they are only sold as "replacement collar with an integrated GPS transmitter and VHF antenna".
It's Garmin part number 010-11130-00. You can find it on Amazon for $42.00.
I broke the superfine coax cable (twice). It gave way on a rough landing. Of note the symptom was that the direction to the rocket was way off, though the distance to it was reported accurately. (Go figure; maybe Adrian of Bdale can explain this?)
Anyhow, anyone with an old DC-30 can find new GPS transmitter units.