red,
Here is some more data:
Jaguar Inboard Rear Brake Info | Coventry West
Centric seems to be common; here are some illustrations and items for different years:
Jaguar XKE Brake Caliper - Brake Calipers - Centric A1 Cardone - 1969 1974 1973 1970 1972 69 74 73 70 72 - PartsGeek.com
Here's an upgrade similar to the one I use (there are others if you use internet search):
17-1214 - Vent Rotor Kit Rear Rotor Conversion
Here is an expensive place to get them; are they really better? I don't know:
XJ6 Brake Caliper, Jaguar Parts Catalog
And another supplier:
Welsh Enterprises, Inc - Jaguar XJS Brakes Rear Brakes Parts
If you're interested in top of the line stuff, I'd give these guys a call:
Team CJ Jaguar brake upgrades
After you have seen what's out there, maybe then you can decide which way to jump. As joy said, the parts are out there too. Rebuilding your own is also an option - if your car uses the Jag handbrake it can get, um, interesting.
Tom