Have you tried Coventry Auto Components in England? They are XK parts specialists and aside from after market reproduction parts they also carry used parts. Also, Welsh Jaguar Enterprises in the U.S. similarly carry a selection of used Jaguar parts and might be worth a try. Good luck.
Sellers probably want to sell it with the stub axle because it is so difficult to extract the axle without a hydraulic press.
You can use one from a Mark V saloon, but not Mark VII, VIII or IX because the steering angle is different. Nor any of the small saloons, also different. Be sure to get the left side not the right side because the alignment flats are on opposite sides.