Colonial girl
Yes mostly they all buy from the same supplier. Here is how it works.
gasoline is produced at refineries, distributed via pipeline, and delivered by semi-truck.
There are a very limited # of refineries, and no new ones have been built in years.
This US energy website says NO refineries in Florida or Georgia, 3 in Alabama
https://www.eia.gov/dnav/pet/pet_pnp_cap1_dcu_SAL_a.htm
Pipelines take years to put in place facing many environmental and regulatory challenges.
Trucks The largest road tankers in the US can deliver just under 12,000 gallons of fuel.
A gas station's fuel storage tank ranges from around 12,000 gallons to 24,000 gallons of each 87 and 91 octane per station, not per pump
A typical arrangement would have one tank dedicated to regular unleaded, while the other tank would be partitioned internally and hold both premium gasoline and diesel.
The 2 x 20k tank arrangement has adequate inventory to last between deliveries, and plenty of capacity to take in full truck loads. (Full loads are essential for minimizing trucking costs.)