If you want the large bottles, not the DIY ones that about 30cm long, you have to go to a BOC agent. They charge you around £40.00 per annum for the loan of the bottle ( you have to enter into a contract) then you have to pay for the argon/co2 or other gas depending on what you are welding. They will not fill any other bottle. You will need a regulator and a special wrench for turning the gas bottle on and off. There are other suppliers but as far as I know they all operate the same way these days.