Thanks Mark. They do come up for sale (mainly Ebay) from time to time but what they sell for is anybody's guess!
One went a few weeks ago, a mint condition size 6, for £130+ iirc.
The best thing to do, although I hate them:D, is car boot sales. A contact told me recently about picking up an older pattern, good sized SAS dpm windproof in great condition for £1! Of course I immediately told him what a s*** he was!