I looked up gas bottle suppliers in the yellow pages and found one that supplied bottles and gas.
I had to rent a bottle for about £25 per year and the gas was £15.
The CO2 is food grade (as supplied to pubs for use with coke and beer).
Bottle size is about 76 cm high and 15 dia and holds just under 6.5 kilos of gas.
You do need a regulator for the connection (approx £15).