Alan, yes im pretty sure you can use soda stream bottles as buffers,

Another option would be a disposable mig welding co2 bottles, Take the 1/8th fitting of the top, drill out a larger hole and push about 25mm of steel pipe in...Drill out the pipe and weld it to the bottle, then tap the pipe to 1/2 BSP...have it certified and that should be enough.

How many liters of gas do you need your buffers to carry?

regards
Dave moulds
Team Turbine/PLF