Any place that sells kegs of beer should also have tanks you can rent for a onetime fee and a deposit. Then just have them refill it, they will also swap the tank out once the safety stamp has expired.
These guys fill CO2 cylinders but don't sell them. I've called a bunch of welders and brewers, and none stock this. Has anybody from NYC actually bought one somewhere that they can recommend?
I went to a welding shop in Brooklyn. I dont remember the address, but if you go to smith street in carroll gardens, and take it all the way to the end, in red hook, there is a warehouse like place, a bit spooky, and you can get a cannister of it, and trade in the old ones for filled ones. May a google searh for it will show it on the map.