If you're a second-hand lover, Ebay always seems to have these loose items for sale, at good prices.
If you're like me, and you want spanking new virgin gear to put on your car, there are a few dealerships like Jaguar Parts International (California) or Jaguar Parts Unlimited (near Boston I think), who will take orders online from anywhere in the US.
You just need to give them the Jaguar part number.
Since they are authorized dealerships, they can dispatch it right from Jaguar's own warehouses and it will deliver to you within a week. Since they're authorized they can even confirm a part will fit your vehicle if you give them the VIN number before ordering.