As far as I am aware there are not any original windscreen wipers with a chrome/stainless steel finish for the XJS. I do not know if you can get retro/universal wipers fit the XJS, but obviously you would have to measure the length of the wiper and the diameter of the flange area. Maybe if you take your car to say Halfords to see if there is an equivalent setup you could use. My comments about availability is based on that there is no mention of them in the original part catalogue and that I have never seen any fitted. When I first acquired my car I did a search myself for some.
There is a stainless steel holder for sale on eBay for just short of £49 and it states it is for an early XJS. I think the issue here is the pre HE is different from the HE models so sorting out one that fits a later model is going to be difficult and potentially expensive. I would like mine in stainless steel but it is at the bottom of the list for improvements to my 1983 XJ-SC.