I'm not sure about Houston, but in Austin, Airgas doesn't carry beergas on hand. I actually had to call around and finally found a place - 'Round Rock Welding Supply' that swapped them out for a reasonable price. Note that beergas 75%/25% is a proprietary blend so you'll most likely be looking at 70%/30% like I found.
Kegged 10 taps - Citra Pale Ale, Session Saison, Blonde ale (Nitro), Cascadian Dark ale - X2 variations, German Pilsner - X2 variations, Vienna lager, wheat, Bourbon bbl aged RIS .
Primary - Lacto/Brett Saison, lacto/Sacch Saison, Lacto/Sacch Sour Stout.