Japan will be selling additional Wii U GamePads at retail; however over in the US players will need to contact Nintendo support.
Speaking to Shacknews, NoA's PR director Marc Franklin confirmed that players won't be able to pick up additional GamePads at launch as it may cause confusion amongst general end consumers.
Your average Joe may assume a second pad would be needed, say in a family with two children, and there currently are no announced titles where it can can be used. These dual-GamePad titles may spur Nintendo to ship separate controllers at somepoint however. Those who accidentally break or lose their GamePads will have to pick up the phone to Nintendo it seems.
Regular Wii accessories - remotes, nunchuck, sensor bars etc will still be readily available in a myriad of colours.
Nintendo of Europe have yet to comment, but it'll likely be a similar affair.