There is a Spark store in the international terminal. As you exit customs inspection into the arrivals hall it is roughly 45 degrees to your left and about 20 metres away, next to the doors to exit the terminal. The Spark store is right in the corner. There is also a Spark store in Queenstown, but not in the airport.

Auckland International terminal map
Pinky blob with number 3 at the far right is the spark store, next to the vodafone store.