Bruce, machine shops make propeller shafts for all kinds of equipment. They’re not overly expensive and you probably will not find one secondhand. It’s a precision part and, having replaced a couple over the years the only way to really get it done right is to have the old one in hand along with the coupling that holds the shaft onto the engine.
The diameter and length are important, of course, but so is the taper on the prop end, the depth of the key way on the engine end, and the fit between the shaft and the coupling. If you’re replacing the prop then you should almost certainly also replace what we call in the States the Cutless bearing, but that you might call a “stave bearing,” which is the metal tube lined with neoprene rubber that goes through the strut and keeps the shaft from flailing just forward of the propeller.
It can be very difficult to get the old shaft off of the coupling, but that can be done with the boat in the water. Once that’s done it’s easy to get the shaft out of the boat, even if the boat is in the water, UNLESS it hits the rudder, in which case the rudder may have to be removed. Once you have the new shaft fabricated you’ll almost certainly need to haul the boat to swap out the Cutless bearing and drive the new shaft into the coupling.
