Try Dale Photographic in Leeds or Speedgraphic, both of whom often have stock when others don't.
As for people trying to charge a premium, that is simple business, when demand outstrips supply sellers can get higher prices.
As of 10 minutes ago Robert White had a stock of 2 Lee Big Stoppers. It was 3, but it's 2 now that mine is now on it's way.
Thanks for the heads up for them.