Keep your carbonated water close at hand - don't run out of gas! You can use these for a trade-in discount credit when exchanging for new canisters. You can bring an empty co2 canister into local stores like Ace Hardware, Best Buy, Target, Walmart, Fred Meyer, or many other retailers to swap it on the spot. These are the threaded screw-in style (with blue cap), not the pink quick connect ones.
Buy in bulk for discounts! 3 for $25, 6 for $40, all 12 for $70.