I replaced one of mine by taking the old light out of the coach and matching the ballast up at the hardware store. We have McClendon's in my area which I would describe as an old time hardware store, not a box store. The box stores may have it, but I never looked there. I, also, had a tube end, that was burned looking, on that unit, and found a replacement for it too.
This keeps all my lights looking the same until I'm ready to change them all to a better light.
Ken