Who knows indeed? Especially since they don't seem to be easily available over here. Thanks for the tip though, I might keep an eye out and snaffle some if they show up somewhere.
I'm currently making do with dressing pads and tape, which isn't convenient, but stops the rubbing (both on the original blister and the new patch of manky skin).
I'm not sure what I'd do if I developed a latex allergy, probably cry, since it would make LARP very difficult.