Actually if you are an owner of the Japanese HG and SS, you can buy Coins in the Counter in the Game Courners located in both Celedon City and Goldenrod City. But if you own the non Japanese version of HG and SS, you have to earn Coins by playing Voltorb Flip, which can take a while.
You have the ability to put notes of what you think the tile may be, so you have a reminder of what a Voltorb card may be. This makes it easier for you to get the coins you need to get the prizes.
Here is the Link with more information.