Question

Where can I buy Dive Balls in DPP?

Answer 1

Non-repeatable
A small islet on Route 223 (Platinum only)

Repeatable
Randomly given by a man in Solaceon Town by showing him the Pokemon he requests (Repeatable daily)

Dive Ball (Bulbapedia)

Answer 2

Sometimes the reporter at the Pokemon News Press will give you three Dive Balls in exchange for bringing the Pokemon he wants to see.

Answer 3

You can get Dive balls from the guy in the house next to the pokemon center in solaceon town. if you show him the pokemon he wants, then he will give a few pokeballs and he may give you dive balls. And it wont help you to catch those pokemon. Only the water type pokemon. or

You can get Dive Balls from the Newspaper Person in Solaceon Town who asks to see a specific Pokémon every day.