BOC tend to give them away for nothing if you ask nicely.
About the only thing that is free, although they have sweets on the counter in the Leeds branch - fill yer pockets!
I too forgot to pick one up when getting the gas. Used a 7mm spanner, but that broke when turning on the Argon (valve was super tight)
A 3-way radiator key worked perfectly, and has the advantage of having square holes, and is brass, so if something is too tight, then it will give rather than mashing the valve.
When I eventually have to get a refill (big bottles are great) I will get a proper key.