Maybe this is a really dumb idea. But the other day I got to thinking. Here I am buying 2 liters of pop and drinking them so that I'll have a bottle I can put yeast into to make co2. But what about just hooking up the pop bottle itself full of pop?? That's full of co2 already with no mess! The obvious problem firstly would be that all the co2 would leave really rapidly so in two or three days you would have none. But you could put a check valve on it the same as a cylinder would have to regulate how much is allowed out at once, to make it last longer. The other issue would be cost, but when you figure out how much it costs to buy 2 cups of sugar to put into your yeast plus all the hassle, mess, and inconsistent results, maybe it would be worth it. How long do you think this would last?? I might just try this for fun.