Usually someone in one of the the clubs makes them, but he's not been able to for a while I believe, so they're becoming rather in short supply! I bought two second hand ones for £7.50 each, I think the new ones might have cost £10, but I am not sure.
However, the specifications for the show pens are in the NHC handbook, which can be downloaded
here, so if you are a handy person or know someone who is, you can make them yourself. (Unfortunately I am not, otherwise I would have made my own a long time ago.)