If you mean, the latch mechanism on the door, SJ Sports Cars seems to have at least the right side (
https://www.sjsportscars.com/parts-and-accessories/A079U4628F.htm) but you might give them a call to see if they have the left side. They are sometimes also known as strikers.
If you mean the part that is screwed into the vertical part of the door sill at the back, those are pretty readily available. SJ Sports Cars has them (
https://www.sjsportscars.com/parts-and-accessories.htm#searchresults) but they are pretty readily available most places. They are known as strikers.
I looked around a bit and was unable to find them on another car. I would suspect that classic Jag parts places might have them since the door handles are the same as those on the XJ6 and XJ12, but that's far from a guarantee.
Don't forget the other usual suspects: DBE, r.d. enterprises. Sports Car World used to have a lot of used parts. I don't know how to get a hold of them - if they are still around. Chuck Nukem mentioned them so he might know their status.
Finally, since Richard has built custom customer cars, he would likely know where to get them. Try calling him or Lotus Supplies.
Good luck!
