Well, since it's Platinum we're talking about, the answer varies depending on the version you're working with. I'll assume you're using the European version, in which gambling is out of the question. You just have to keep checking the slot machines each day until you have enough coins, looting unused machines for scraps, now that's something to teach the kids. However, if you talk to the game corner's receptionist, she will occasionally give you one of the TMs the Game Corner offers in their prize corner. So keep talking to her until you get the one you need.
If you're using a different version with gambling included, we'll need someone with experience on these gambling games because I have none.