No! You can buy tires anywhere you want. Those sizes are the best sizes for your 2004 NSX.
The stock tires for the 2002-2005 NSX (Bridgestone Potenza RE040) are no longer available. The best, stickiest tires you can get in these sizes for street use are the Dunlop Direzza ZII and the Yokohama Advan Neova AD08R. Both are outstanding; the Dunlop is slightly better on dry pavement and the Yokohama slightly better on wet pavement. The Falken Azenis RT-615K is also very sticky and very good. Those who are looking for a bit longer treadlife and are willing to accept slightly less grip in exchange, may want to instead consider the Kumho Ecsta LE Sport, which is still excellent, just not quite as sticky as the previous group. The ZII, AD08R, and LE Sport are all available at the Tire Rack. Discount Tire Direct, Discount Tire's website mail order arm, sells the AD08R and the RT-615K. Costco only sells tires from Michelin, BFGoodrich, and Bridgestone, none of which are all that great for the NSX, and does not sell tires in these sizes anyway.