Soda stream and just rent a 20 or 40 pound tank.
http://www.co2doctor.com/freedomoonespec.htm
We use one at work (regular soda stream) and the only problem we've had is sanitation. People drinking out of the bottle, then putting it right back on the machine. A box of alcohol prep pads now is right beside the sodastream. You jsut have to remember to rinse off the bottle before you drink out of it.
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