Buy it? We'll make it!
You can make one, absent this information. Just ensure that it plugs into the rudderhead adequately (not too much play) and can be adapted to block-and-tackle or some kind of extension. To accommodate this it's absolutely permissible for it to attach off-center or at an angle (to clear the pedestal). Nine out of ten emergency tillers either are too short or provide no means for further purchase. Think about the conditions under which you'll lose a steering cable and then decide if you want to steer the boat with a 2-foot metal bar!
In the event you have to use it, removing the wheel might be a good idea (or necessary). Now consider how many popular helm wheels are too big in diameter to fit down the companionway hatch (--doop!).
Keep in mind also that the material of the emergency tiller has to be strong. I have seen 2x4s employed for this purpose; but really something laminated is best. Steel is fine but heavy and not so easy for the average owner to acquire or make. A single hunk of wood can be stiffened with unidirectional 'glass adhered with epoxy. It doesn't have to be pretty; it just has to be strong, quick to install, and available!