To get the harbor mail you need to go to slateport city
You can buy Harbor mail at Slateport City
slateport in the store.
go to the harbor at slateport city.
he is in the slateport harbor, right above the Pokemon center
Use the boat with mr. briney to the left of petalburg to get to dewford. Then, after you deliver the package for the devon people in the cave, you can boat again to slateport.
In the place where the SS tidal is.
slateport city Pokemon mart
From the PokeMart in Slateport
Slateport city Mart
You can find an harbor mail in Slateport City Hope I Helped By Matthew Teves
In slateport city but this is only in ruby and sapphire and emerald.