You buy harbor mail and give it to a girl she will give you a coin case.
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You get the coin case in the house next to the game center in mauvile town.The lady will give you it.
trade harbor mail with a girl in mauville
get a harbor mail from the trick house then trade it in to the girl in the house right next to the mart in mauville city
go to your mom after getting your 5th badge this works for both sapphire and ruby but am not sure about emerald !!!!!!!!!
next to the pokemart in maville is a house a girl says she wants a harbor mail u can find it in both the trick room and in the mart in slateport u can either go through the trick room and find it or u can buy it for $50 in slateport once you give her the harbor mail she'll give u the coin case hope this helps :)