McMaster-Carr have 5 different versions of 1/2-20 UNF LH dies, starting from US$2.54! (From their home page, go to the "Machining and Clamping" area and click on "Dies", then 1/2-20, then Left Hand Thread.)
Unfortunately, McMaster-Carr have gone funny with the War on Terror and will no longer ship overseas, only continental North America. That's a shame as their catalogue listings provide hours of fun searching. Perhaps someone here on the Forum will be able to help out with shipping?