For heavyweights, 2kg cylinders are the common size used (usually one 2kg bottle as a supply and one or two modified bottles as buffer tanks) and for featherweights, 600g/20oz paintball bottles tend to be the go-to option. The key point is if you can get them with screw valve handles on them rather than lever ones; this will make it easier for adapting the valve to take an M10 nut which will allow you to use a 17mm socket tool to turn on the gas.