The soda fountains at restaurants and bars have all gone over to Liquid CO2 in big Dewar tanks, low pressure supercooled liquid, delivered by tank truck every week or two. Which means you can buy used CO2 pressure bottles for cheap - it also means once you buy the Dewar you don't need to go get it filled, you can arrange for route delivery to your shop on a regular schedule to top off your tanks.
They make dedicated CO2 detectors for parking garages to kick on the exhaust fans, and you can use one for the same purpose.
And while you're at it, find a carbonator pump, a small icemaker with space for a chill plate at the bottom, and the water double-backflow preventer so you can set up a 4-button Bar-Mate Gun and 4 Bag-In-Box pumps. You'll have to get Regular Dew Syrup, the Doc's Special Blends probably not... The beverage gas delivery company can probably source the syrup too.