Here is a link to look at Porsche dealer inventories. Unfortunately you have to go state by state and dealer by dealer, but consider it a treasure hunt.
http://www.porschedealer.com/dealer/cgi-bin/locator_us_CI.cgi
Personally I've had great success with Woodhouse here in Omaha, Ne and Boardwalk in Plano, Tx (suburb of Dallas), but they may not be located exactly where you are looking to buy. I'm assuming you'll want to look along the east coast. Hope this helps.