Hi Dr. Walls -
Thanks for your feedback on the product. We are actively working to expand our retail presence, and we are hopeful that very soon we will have abundant locations to pick up exchange carbonators.
Meanwhile, the manufacturer's website is a great place to order exchange carbonators. The Fountain Jet starter package listed here by SodaStream comes with 2 of the 60L carbonators, and the current price to exchange these is $26, plus shipping (which varies, depending on the delivery address.) This comes out to 22 cents per liter, before shipping...which might add another 4-5 cents per liter. We do encourage people to purchase an additional carbonator, so you can have one in the machine while the other two are being exchanged (yes - you can exchange just two at a time now...great new improvement to our web ordering!)
As for the carbon footprint, shipping 1 cylinder of gas is equivalent of shipping 60 liters of sparkling water. Even though there are certainly transport miles associated with shipping anything - the SodaStream system cuts down these transport costs by over 90%.