The Softbank outlet in Narita has only limited variety--rental and prepaid cellphones for tourists to use during their stay in Japan. No iPads. Apple doesn't have outlets there.
iPads are sold only at Softbank outlets in Tokyo and other large cities, Apple Stores (in Ginza and another location I can't recall) and at large retailers like Yodobashi Camera and Bic Camera.
I'm also dubious whether you can get an iPad just by dropping in at a shop, considering its popularity here. Chances are high that they are out of stock. You'll have to make sure the shop has one to sell you.