I actually have CO2 cartridges for a seltzer water ... pitcher ... thing? Pour water in, close it up, screw in a cartridge, instant seltzer. It discharges all the CO2 at once, though, so it's not so useful for this use case. I didn't know about the bicycle pump, though -- that would be a great dispenser, and way easier than screwing around with baking soda and balloons! When I'm back in town I'll be sure to check out a bike shop.