In Solaceon Town, there's a building known as the Pokemon News Press. In there, there's a daily event you can do to receive free Heart Scales and Pokeballs! This task is to show the man in the glasses a Pokemon by the time the day rolls over to tomorrow. Doing so, as I said above, gets you a random Pokeball, except obviously Master Balls, Premiere Balls & the other types of Pokeballs you can't obtain (Park Balls, Cherish Balls and Safari Balls). You can only get Heart Scales in Platinum, though.
Did you do this small daily event in Pokemon? I was never able to show the man the right Pokemon at the right time, sadly. He always just happened to ask for Pokemon I didn't own.
I remember completing the task a few times when I first played Diamond and Pearl, although I never bothered to do it in Platinum version. I liked that we could get certain kinds of Balls from the newspaper editor, but overall it wasn't something that I did repeatedly.