Saw this on bargain alerts on boards. No idea if it may be useful, but may be:
http://www.decathlon.ie/co2-cartridge-16-g-en-s167162.html
16g CO2 canisters for €1.06
I don't think three CO2 canisters for tyres are suitable for beer?
Nope they are cheaper than food grade canisters used in soda or cream siphons as they use oil to form the canister which will taint your beer