The parts you are looking for are called the saddle (attaches to the tiller) and the universal (attaches to the extension). The saddle has to fit the curvature of the tiller, so you should buy one for the tiller that you have. Presumably, a wooden Sunfish tiller.
There are several brands manufacturing these items and the parts are not always interchangeable. Two major brands are Ronstan and RWO.
Personally, I would buy from Annapolis Performance Sailing because they carry the Sunfish specific parts (and give good service).
PS: Universals will break eventually due to wear and tear. One speedier way of doing this is hanging on to the extension as you go swimming .
You can buy a universal with a rope inside (more expensive) which allows you to still move the tiller with the extension once the universal breaks. Nice but not required.