They used to have a large canister version, but Sodastream stopped distributing the larger canister. I use mine daily, up to 4 bottles per day, and 1 canister lasts me around a week. Considering I’m making almost 1 US Gallon per week, it’s worth it, and you can exchange the canister at Target, Bed Bath and Beyond, and other small retailers.
Short answer: Bed Bath and Beyond or Target. There’s really little difference in price between retailers. I would recommend the manual carbonators over the fancier electric ones. Just more to break, IMO, and these are really very simple devices. No need for frills.