Pete;
I ran a search for 1/24 scale diner's. I remember seeing one from Monogram as part of a kit. Unfortunately, it is a Die Cut (Cardboard) addition to the kit. So not exactly what you were looking for.
If you can locate the Kalmbach How-To series for Car Truck modellers there are diorama specific books and you're sure to find a diner diorama in there.
It would not be that difficult to build yourself either. It's a fairly simple structure and can be built fairly quickly.
Not exactly a full answer but hopefully a start.
Cheers;
Gregory