Where I live, even with seatbelt laws, a Nuclear Meltdown would be more preferable to most people, then seatbelt status lights. Although I'm surprised the insurance mafia lobby has not tried to get that as a law. In this state, a seatbelt violation is not primary cause for a traffic stop. Simply because the greedy municipal or Mayor's traffic courts (Owned and Ran by the City) would love that.
I've seen those fixtures in an Automotive environment, years ago. I do not know who makes them. So I do a search for Green Warning Lights for Doctors, as I thought we had that on the books here, and I find "Feniex Industries " beacons.. The T3 in a single or the Fusion Dual Stack Looks very much like what the OP is looking for.
Search Terms to try : TIR LED Surface Mount Warning Light Heads
Steve