Autotrader enables you to set a search by your own criteria. If the car you want is not there, you can save the criteria and the site will e-mail you when a car comes up which matches your requirements.
This is very valuable as it can suddenly include a car when the price is reduced to speed up the sale. Saved me about £4,000 on my S Class.
With Turo, you can book and drive any Mercedes you want, wherever you want it, from local hosts across the UK, US, Canada, and Germany. Test drive cars you otherwise might never get a chance to try, or hire something interesting for your next holiday: www.turo.com