The North Pole is the northernmost point in the world.
It is impossible to buy 'anything' at the North Pole; you should bring 'everything' you might need for your expedition. A small variety of beer, vodka, and whiskey is available in the Camp Barneo dining tent, at reasonable cost given the location (e.g. $5 for a beer, in 2013), as are souvenir t-shirts, until they sell out.
The North Pole's ocean-formed ice contains salt, making it unsuitable for drinking, even when thawed. BYOB. Celebratory champagne is traditional; vodka at -40°C while standing on an iceberg is also quite appropriate.