Weber do one but it's not cheap
http://www.wowbbq.co.uk/products/weber- ... 22274.htmlAlternatively you could buy a stacker (something like this
http://www.barbecuestacker.com), and then adapt it yourself to take something like this
http://www.amazon.co.uk/broil-master®-BBQG04-ZBH-02-Rotisserie-Spit-Grill/dp/B008YA34OW/ref=sr_1_4?ie=UTF8&qid=1420278023&sr=8-4&keywords=bbq+rotisserie+spit
Still not cheap, but cheaper than the Weber one with the added benefit that you'll be able to use your kettle as a smoker
Finally, rotisserie systems do come up on e-bay from time to time.