Jaguar XKE cars were manufactured between 1961 and 1975, so you will need to purchase one from a classic car collector. You won't find one at a used car dealership.
Most classic cars are not available for purchase from typical local dealers. However, you can find them by finding a dealer that specializes in classic cars
www.GreatWorkshopManuals.com (Workshop Manuals for Classic & Exotic Cars)
There are many companies which sell genuine Jaguar parts for a classic car. You can try such websites as Cherry Hill Classic Jaguar, Jaguar Classic Parts and Auto Parts Warehouse.
You can purchase a Jaguar XJ8 at a local Jaguar dealership, or you can find available Jaguar XJ8s on the sites Auto Trader or Ebay Motors and purchase from the sellers.
Pontiac classic cars are more usually found from classic car dealers. Occasionally one can find them through looking in Autotrader or on eBay Motors, but care should be taken when buying a car through an unknown dealer.
You can find reviews for the Jaguar S Type cars on the official Jaguar website. They can also be found on many car enthusiast websites and blogs to read.
One can purchase a classic BMW from many different car dealers. One can find car deals using online car finding tools such as Car Dealers, Cars, and ClassicBMW.
It depends on the country. In Australia, for example, classic cars are quite common. Rallies showcasing classic cars are often held on weekends, and it is easy to find them in classified advertisements.
used
nadaguides.com/Classic-Cars
http://www.nadaguides.com/Classic-Cars