There is a reason it is hard to find. If both ends are threaded it can easily come apart due to vibration. One end threaded and the other silver soldered, "Z" bent or with a 90ยบ E/Z Link is better.
Here are shorth lengths for attaching to the end of Sullivan hollow rods.
http://www2.towerhobbies.com/cgi-bin...?&I=LXFV02&P=7