Recently, my XKR was sidelined with a dragging left front brake caliper. I suspect that a torn dust boot caused the problem letting in dirt and moisture. NONE of my local auto suppliers could get this caliper or caliper kit for me. Then upon seeing what the dealer wanted for a new caliper, I decided to go around that idea. What I eventually found was that the single piston caliper repair kit for a 1990 BMW 535i fits perfectly. They both 60mm bore calipers from the same mfg. About $5 at Auto Zone for the seal and the dust boot per wheel.
However, if you are needing to rebuild your caliper and replace your caliper piston due to excessive pitting, you might order a reman caliper assy for the same BMW and remove the piston from it to replace yours. The piston will be about 1/4" longer. It might work if your pads and rotors are sufficiently worn, but I didn't try it. Instead, I turned it in a lathe to exactly match the old one. This was $44. + $52 core charge, and I also got the seals, slide bolts and new bolt rubbers & covers.
If you haven't rebuilt many brake calipers, AND brake systems, I suggest seeing an expert to perform the work for you. Shortening the piston and getting the piston in the seal and dust boot isn't always as easy as it sounds. Just changing the dust boot requires you to remove the piston also. Any loss of braking can be fatal to you and others. I am merely pointing out the fact that the overhaul repair kits are the same when you cannot find a Jag listing.

 

Thats useful info..... I must admit I had trouble finding a piston once and I turned one up complete out of stainless steel, luckily I managed to get a seal kit easy enough

 

My first idea also was to make that piston from stainless bar. That shouldn't corrode or pit I thought. I sure didn't want to go thru re-chroming my old one yet. However, I didn't have a big enough chunk of the stuff here. It should be about 2 1/2" by 2 1/2" bar stock to cut it from. So I went to visit my favorite machine shop.

They quickly refused citing product liability lawyers here in the USA. Whether they were right or wrong, that didn't help me so I went to plan B. I began searching cross references online starting with the seal kit, then the calipers related to those kits zooming in on the pictures to come uo with the BMW one. There were several German cars (3) using the same caliper kit at very reasonable pricing, but the 535 seemed to stick out for some reason to me. You could see that the castings were the same, but the porting was reversed with different threads. Didn't matter since I was re-using my old housing, because I didn't want BMW cast into the caliper body.

 

Nice add Marc. Other XKR owners, if not soon, will eventually thank you for the alternative.