You actually have the good one! Yours is an Electrolux motor where earlier models had a Lucas motor. My car is an 87 and the Lucas motor self destructed first by the cheap plastic shroud disintegrating and then the motor rusted and the magnets came unglued and broke inside. My repair was to find a 1990 XJS unit to replace it with. It is an easy swap since it just plugs right in. I used the 90 grille that was already with the newer replacement even though it is flat where mine was glossy black. The only issue I am left with is that it will not automatically park...but with a swap of relay and a snip of one wire mine should then be able to park. I haven't done the alteration since it is simple to manually park them and I don't need the intermittent mode.
Hopefully someone will benefit from that info. Now to your question.
Junk yards, ebay and some online used suppliers that will charge you an arm and a leg for one. Try Craigslist, Ebay and the junkyards unless you have deep pockets. I am not aware of any new ones being offered. You can also take the motor to a motor rebuilder to have yours rebuilt, if it is rebuildable. Mine was, but they wanted more than I could by another car for.