You can earn coins in-game by beating enemies, completing Turtle Time, destroying the boss, through the Pizza Wheel.
You can also purchase them from the Coin Store using real money.
  1. In the Turtles Lair, Tap the Coin icon
  2. Select one of the coin bundles
  3. Login to iTunes account to spend real money for a coin bundle or have your parents log in for you to purchase extra coins.