I got lucky, I found one in a Gran Coupe Cuda in a junk yard around 1980 for $5, they didn't know what it was. Went right up to the dealer an bought a new dome light assembly for $15 and had it re-upholstered for $60.
I hooked my fasten seatbelts light to the timed light for the ignition (I have the light package), the door ajar to the dome light and the low fuel, well didn't work.