I came across this on E-bay of all places, also there is a thread here dating back to 2006. Does anyone still use this with success?
"The most important step is the mixture that provides the CO2. For two-liter bottles, our favorite mix is:
6 oz pack of Jello (your choice of flavor)
2 cups boiling water
1 cup cold water
2 cups sugar
1/2 cup lukewarm water
1/2 tsp Active Dry Yeast
Dissolve the Jello in the boiling water, add 1 cup cold water and pour into 2 litter bottle, place in refrigerator overnight. Next day add the yeast to the lukewarm water to activate it, then pour into the 2 litter bottle and cap. This will start producing CO2 in a few hours, do not shake the bottle or you will get too much CO2. Each bottle will produce CO2 for about 3 weeks. There are also other formulas for CO2 production, the Jello mix has proven to be the most consistent we have tried so far."