Hobbyweld supply a range of gases including food grade Co2. A 6kg cylinder will last you over a year, do be sure what supplier you are going to use, if its coming from a mixed gas pub cylinder it will have a different regulator connection than a bog standard Co2 tank though you can get an adaptor which iirc is argon to Co2.
You could also try Air liquide, some suppliers charge a bottle deposit some don't, it is well worth researching for as many suppliers as possible and there costs like either getting free delivery or is a saving significant enough to justify a 100 mile round trip to collect as well as refill costs or bottle deposits.
