Buying a Sodastream helped our family save big on soda water, reduce our plastic waste-stream, and resulted in us providing a steady revenue stream to a sleazy company that used to run a factory in the Occupied Territories whose business model relies on patent abuse in order to sell you compressed CO2 at a markup of several thousand percent.
There is an alternative!
Buy an $18 adapter and a a $20-25 tank and you can buy your CO2 from any vendor you choose (SCUBA/sporting goods stores, as well as welding shops, paintball shops and homebrew places will all fill your tank for pennies, versus Sodastream's overpriced $15 air).
The replacement tanks are sized to fit whatever model Sodastream you've bought, and are made from "food-grade" materials, so you don't have to worry about lead or other contaminants in the canister or its fittings.
I just ordered three bottles and an adapter for our house, and I'll be calling the SCUBA place up the street when they open.
You'll need to do two things to get set up: order the hardware, and call around to figure out where to get the tanks refilled.
The only snafu I ran into was finding a place near me to refill the tanks. The closest sporting goods shop (the first and easiest place you should try) was a 20-minute drive away. A paintball shop was half an hour away. Get clever, however, and you'll eventually find something—a welding shop, a dive shop, a home-brew store, a nice guy at the local Airgas who knows where you should go. (Depending on where you live, picking up a $10 adapter that puts a standard CO2 tank attachment on a paintball bottle might give you more options.)