Hi all,
Most any good hardware store will have the bolt you are looking for. It can be bought in square head, although 1/2" square is most common now while the TR original is 7/16" square, or in hex head, possbly even an Allen or socket head.
Just go to the plumbing department and look in the pipe fittings. You need a 3/8" NPT fitting (I know, seems much larger than that... plumbing pipe thread fitting sizes are referenced to the inside diameter of a pipe using this thread, not the diameter of the thread). I use brass, since it's less likely to damage aluminum parts and seals well. My local store also offers them in galvanized steel and plain steel.
I've been looking for a set of square wrenches too, if anyone knows a source.
Cheers!
Alan