The cylinders are available online and in retail stores. At pricehunter.co.uk you can also find cylinders from manufacturers such as SodaStream or Cago in different sizes.
Check with the manufacturer of your water carbonator to find out which cylinders are compatible with your device, as not just any cylinder will fit on every water sprayer.
Some offers of water sprinklers already include one or even two cylinders with the purchase of the device. Again, take a closer look so you know if you need to buy a cylinder separately or if you can get started with your water carbonator right away.