Wheel Enhancements is just about the most expensive online retailer of wheels for our cars that I've found.
I bought my aftermarket 18 inch carrera lights from these guys... excellent Italian workmanship, and they have the correct offsets for Boxsters...
http://www.wheeldynamics.net/
They sold me Michelin PS2's with the rims for a decent price.
BTW, reconsider buying 19's. They can be heavy, and the rubber choices are far and few between in the speed rating we need on our cars. Then there's the price of rubber. Yikes!
But it's your money and your car. Enjoy whatever you choose.