I managed to find a centre arm with them in on an early ES model. Fortunately it had the same colour leather in as mine so just swapped the whole arm over. It looked to me like it would be difficult to retro fit them and do a decent job of it, getting the leather trim neatly finished etc.
TBH they are totally different vehicles from the ground up. You are just as likely to get an armrest from a different manufacturer to fit. You'd be more likely with something out of the Rover stable, like a 75 or summat, but still unlikely.