The Tap & Die Co.
An excellent source for taps, dies and other metal cutting tools in those hard-to-find sizes. These guys seem to have it all - I've never found them wanting, and I've ordered some pretty obscure thread sizes from them! Since they're located in the London, England area, most of us will have to deal with them through their secure on-line store. However, their on-line service is both super-fast and efficient, plus their prices are very competitive. The quality of their products is also extremely good - I have yet to receive a dud tool from them. If you can't get a given threading tool locally, this firm can almost certainly help. Check them out!