1. Buy them from Poké Marts. Poké Balls are sold in every Poké Mart in the game. The prices of the Poké Balls increase as you progress through the game, but you can buy as many as you like.
2. Pick them up from the ground. Poké Balls can sometimes be found on the ground in certain areas of the game. Keep your eyes peeled for these, as they can be a great way to get free balls.
3. Receive them as gifts. Some NPCs in the game will give you Poké Balls as gifts. For example, the Poké Fan in Sandgem Town will give you a Great Ball after you have defeated the first Gym Leader.
4. Use the Poké Ball Repels Poké Balls can sometimes be obtained as a repel through wild Pokémon.
5. Win them from the Battle Frontier. You can win Poké Balls as prizes for winning battles at the Battle Frontier. The higher your rank, the better the prizes you can win.
6. Use the Pokétch's Ball Searcher app The Pokétch's Ball Searcher app allows you to find hidden Poké Balls in the game world. You can find this app by going to the Pokétch menu and selecting the Ball Searcher icon.
7. Use the GTS. The GTS is an online trading platform that allows you to trade Pokémon with other players. You can use the GTS to trade for Pokémon holding Poké Balls.
Using an Action Replay (AR) device to get balls in Pokémon Platinum is not recommended, as it can lead to glitches and other problems with your game. If you want to get more balls in the game, it's best to use the methods listed above.