Radiator Nipples-LH/RH threaded

I am looking for 2 Radiator Nipples for cast iron radiators. These are used to connect 2 radiators together. They have male LH thread on one side and male RH thread on the other for 1-1/2" pipe. They have also been called "Opposing threaded nipples". I have tried Burnham heating and Alternate Heating Systems, but neither carry them in stock. If anyone out there knows of a pipe threader of a manufacturer that might be able to help me, I would greatly appreciate it.
Thanks, David