I've found that CTSC transmission supply is a good source for ZF parts and fluid. Here's a link, I think, Oil change kit for 6HP26/28/32
For twenty bucks a quart, why use anything else? You're only getting some of the fluid out and it seems to me that it's best to replace it with the same product. I made the mistake of not getting a new pan too, but my car only had 55,000 miles at the time and luckily the old fluid was still pretty clean.
I had mine done by a competent, local transmission shop for $150.00 and it turned out fine, installing a new connector too. They didn't have Jaguar software, but did it fine with just a laser thermometer for the fluid temp.
It seems to me that on average changing the fluid and pan every 50,000 miles is a good round number, less if you drive the car hard.
Keeping the fluids of the car full and clean is probably the most important thing to do if you want to keep the car.

 

Mercon SP is one-third the cost of Lifeguard6, meets the required Shell M1375.4 specs, and has provided years of excellent results in many of our ZF 6HP26 transmissions here on the forum. I have no qualms recommending it for any out-of-warranty 6HP26 application....