If you wanna buy true imported tires, with all the duties and taxes paid, and with warranty; I would only suggest Tire Point at Metro Islamabad. Otherwise, you can contact Sheikh Brothers, Pakistan rubber and tire company, etc. For their authorized dealers of Yokohama and Bridge stone tires in the twin cities, respectively.
If smuggled, the market is open for options and competition. You may visit any shop in Blue Area, Kashmir Road, Saadi Road, etc.