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

Update - in 2023 the Tap & Die Co. was acquired by CR Tools of Sheffield. They continue to offer the same range of products formerly available from the Tap & Die Co.  Check them out!