I have never seen a cocking rod that is NOT 10-32 threaded to go into the hammer. So in other words, cocking rods are standard.
If all you lost is the long hex nut part, then you can replace that with two 10-32 hex nuts (any hardware store), smaller the better. First hex nut is the adjustment, second hex nut locks the first one in place. Hopefully you still have that rubber bumper. If you lost that too, then you can replace that will a small plumbing washer (hardware store again) that will fit tightly over the rod and not interfere with the cocking block. If you lost that little set screw inside the hex nut, check your local fastener store and you should be able to get one for less than 50 cents.
If that's too much run around, try my buddy at Jon at Compulsive? $12-15