Tokens can be purchased from the in-game store (tap the token counter at the bottom of the screen to open the store). 

Tokens are also awarded every day you play. Complete one puzzle to unlock the Daily Reward - if no puzzles are currently unlocked and you don't have enough tokens to unlock any more puzzles, one puzzle will be unlocked automatically. 

You may also be awarded tokens for completing puzzles and other objectives in the game.

You may also be able to earn additional tokens by viewing video ads or taking advantage of Tapjoy offers in the Tokens section of the Store (these options may not always be available).