If you don't need a lot of CO2, you can make your own using yeast and some sugar in a softdrink bottle. Around here people will use a 2 litre bottle with a couple of cups of sugar and a packet of yeast. It will produce CO2 bubbles for about 2 weeks and then you relpace the yeast and sugar and it gets going again. To move the CO2 to the water, drill a hole in the bottle cap a little smaller than your air tubing and stuff the air tube through the hole which will give a nice tight fit. Hook the other end of the tube to a porous air stone for a diffuser and you have maybe a dollar invested in a 2 week CO2 supply. Next time it only costs the price of the yeast and the sugar.