For those who want another Wii U GamePad other than the one that comes with the controller, there's not going to be an easy direct way to do it. Nintendo of America director of public relations Marc Franklin has confirmed to ShackNews that the controllers will not be sold at retail during launch.
Those that will want to purchase a new or replacement controller will instead have to contact Nintendo of America customer support. Wii U GamePad controllers will be available at Japanese retail for roughly $150.
While certainly a bummer, there's a good reason behind this decision. None of the titles available during the launch period will support the use of two GamePads simultaneously.
Nintendo confirmed during E3 2012 that the Wii U will have support for two GamePad controllers, despite earlier comments that it will not. While it's unknown when games will come supporting the feature, Nintendo has said it will be sometime well after the launch period.