BUGGER! As soon as I order one for $254, people come out of the woodwork with mint condition ones!
Hah, don't worry it will help someone on here no doubt.
To summarise my findings as at July 2010:
1) these valves are NLA from nissan. They're not making them anymore.
2) Barry (Baz) from datsport (adelaide) may be the batman of obscure datsun parts. He specialises in 510 stuff (datsun 1600) but there is a brand new valve on its way to me now.
3) there is a place in the US, Washington DC, called datsunstore.com run by "zman" (a guy named Oliver). Google doesn't really turn it up but I saw a link on the US zed site. They can rerfurb your valve, but you have to send them the core. Once you factor in postage there, postage back, and the cost of the rebuild, it might be lineball with the one I'm getting from Baz, but a lot more screwing around. But in any case I'll be holding onto my messed up core.
So my main point is, people, if you can get hold of these valves in whatever condition do it because they're only getting rarer. Having a core at least gives you options in terms of rebuilding.