Harbor mail
From the PokeMart in Slateport
you go to the slateport city mart and you can buy it
Mail can be purchased at most marts the lilycove dept store is where you want to go for that for sure.
you go to muaville and get harbor mail and give it to the lady there!
Just go the place where you buy stuff. (The one with the blue roof.)
She is in mauville city in a house right next to the pokemart, give her harbor mail to get a coin case.
Pokemon ruby's version IS Pokemon ruby.
Pokemon Ruby has Pokemon that Pokemon Sapphire does not and Pokemon Sapphire has Pokemon that Pokemon Ruby does not.
go to the slateport poke mart scroll down the item list and at the bottom it sould say harbor mail $50, buy it and you have mail
i dont know where to find it but you can buy it at slateport city`s mart
ruby