Instructions
Insert the "Pokemon LeafGreen" cartridge into the Gameboy Advance cartridge slot at the bottom of your DS. Insert the "Pokemon Diamond" cartridge into the slot at the top of the DS then switch on the handheld and start the game.
Put a Pokemon at the head of your party that knows the "Thief" ability. If you do not have this ability yet, find it in Eterna City inside a Pokeball close to the Galactic building. This ability allows Pokemon to steal items that are held by opponents.
Travel to Route 227 and walk around through the tall grass so you can get into random encounters. If you have the "Pokemon LeafGreen" cartridge inserted into your DS, you have a chance of running into wild Magby's which hold the Magmarizer.
Use the "Thief" move as soon as you encounter a Magby to steal the Magmarizer if holding one. Make sure the Pokemon using "Thief" is not already holding something. If the wild Magby does not have a Magmarizer, just defeat him and keep searching.