Service center chargers typically *are* included on supercharge.info even if they aren't on the official Tesla map. Any entry with "(SC)" next to it's name is like this. Check out these two just north of Atlanta:
View attachment 501583
Note the half-and-half red/black icon for the service centre.
I would suspect that every service centre has superchargers but they might be hidden away from public view or operating at a reduced speed. These ones, however, look like they are out in the open. Are they accessible to non-service centre customers?