Defeat the fifth level (Rock Level) and obtain 100 coins or 300 coins (depending on how quickly you want to gain your free coins). After beating the fifth level with the proper amount of coins, head to the Flipside Store and buy either one Gold Bar or three (for 100 or 300 coins).

With those in hand, head to level 5-1 and go left at the beginning of the level. Enter the Rock Shop you encounter and sell your Gold Bars to the guy there for 15 coins more than you purchased them for. From him you can then buy Puzzle Boxes for three coins and sell them to the Flipside/Flopside Stores for 5 coins (netting +2 coins per box). This is a great way to earn cash in the game.