I've never looked for one. I'd still like to open one up and look at it. There can't be much to it, it's just a 1 way valve (in my mind, anyway) which means someone could easily make / buy a valve that does the same job for a few quid. The pressure and temperatures aren't high at all, so would imagine someone handy with pneumatics would be able to say "ah yeah that's just an XYZ valve, they're the same as.....ABC".
If someone's got an old one they don't want I'll have it and do some investigating. I think mine may be a bit iffy too.
