where you fill, is generally dictated by the tank you already own.
most places won't just take a CO2 tank you bring them, and fill that up. it's usually an exchange system.
if your tank doesn't have the sticker/stamps showing that it has been recently tested, the place is gonna charge you a surcharge for the test, and swap you one of their own.
it's kinda janky, but pretty much part of the business. they ALL do it.
Airgas and Praxair are pretty much all over the place.