How do I Save specific data so that when I close the game and come back all purchassed items return?

I have a game that will eventually be set with in App Purchases where you can purchase multiple ship sprites to use as pawns in the game. Play the game if ship is destroyed one ship is removed from the number of ships owned. It works well until I close the game and reopen and then all ships return to 0. what is the best way to store data like this?