Hi Lee, Top marques & the Sunday Times (mags. & websites) are the best there are loads to choose from, there are plenty of other places (P.Post, Pistonheads website, 911 world, GT etc) but these are the best two. When I got mine (nearly two years ago!!) I decided what spec I wanted, asked a couple of Porsche buyers for a trade valuation & looked to pay a little more privately, many weeks pawing, negotiating, checking. Look for the ones that have been advertised for a few weeks & are realistic on price after all the dealers phoning them & offering nothing!!
Great choosing a new car, stressful but fun, let us know how you get on & if you need any other info.
Regards
Grant