A woman in Mauville City is willing to trade her coin case, which allows you to hold a maximum of 9,999 coins and gives you access to various gambling mini-games in "Pokémon Sapphire." To trade for the coin case, you first need to obtain a Harbor Mail.
Get a Harbor Mail
If you do not already possess one, a Harbor Mail can be bought at the Poké Mart in Slateport City for 50 dollars. A Harbor Mail can also be found in the Trick House on Route 110. It is hidden in the far northeast corner of the second puzzle room. You have to solve at least a portion of the puzzle to get to it.
Head to Mauville City
Once you have the Harbor Mail in hand, travel Routes 110, 111, 117 or 118 to reach Mauville City in Hoenn.
Trade for the Coin Case
After you have arrived in Mauville City, head for the house just to the left of the Poké Mart. Inside, an old lady is waiting to trade her Coin Case for the Harbor Mail. Once the Coin Case is obtained, you can enter the nearby Game Corner and play the roulette and slot machines.