To unlock new Pins you will have to rank up to additional Arenas. At the top of the Versus screen there will be an "i" icon next to the arena name. eg. Arena 1: Thrashball
Clicking this will show you how many Cogs are required to rank up to the new Arena and the Pins that can unlock within it. Once you start getting to higher Arenas you have more options to get Pins that you do not have yet. These Pins will start dropping in Gear Pack Slots and appearing in the Store.