The only Magnet you can get is located inside the cave on Iron Island. You’ll find this item on the far western edge of B2F.
No Hidden Moves are required to reach it, but you must take a ship from Canalave City to get to the island.
There are no other ways to acquire more Magnets in the game (other than trading).
Magnet Location (Step-by-Step)
The Magnet is found on Iron Island, which lies far north of Canalave City and requires you taking a ship to get there.
The cave on Iron Island is full of Rock- and Steel-type Pokémon at levels mostly in the low 30s.
Here’s how to get there and where to find your Magnet:
To locate Canalave City’s harbor, first cross the drawbridge to the west of the Canalave Pokémon Center.
Once you pass the Canalave Gym, turn south.
Pass the houses, and turn eastward at Sailor Eldritch’s house. Speak with Sailor Eldritch at the dock and have him take you to Iron Island.
On the Island, head up the staircase to the east.
Pass the blue-roofed house here, and take the northern staircase further up the mountain.
Head through the entrance to Iron Island’s cave at the top of the staircase.
Once you’re inside, descend the right-hand staircase.
Climb the first set of stairs on the next floor, then head eastward.
When you reach the lone red oil drum, climb down the nearby staircase and head south.
Make your way around the trainer and climb the stairs closest to the cavern’s southeastern corner.
Then step onto the lift to be lowered down to the next floor.
Descend the stairs to the west of the lift.
Note: If you haven’t been through this cave with Riley before, he will join you now, helping in battles and healing your Pokémon after every skirmish. This is an excellent opportunity to grind levels, and to also find the Magnet and all the other items of the cave.
Once you’re down the stairs, head west through the next chamber, ignoring the corridor to the south.
Continue traveling westward past the Hiker trainers.
Head up the short staircase at the western edge of the room. Then move southwest across the elevated terrain.
Just keep going south and at the dead end will be a Poké Ball item on the ground.
Pick up the item to obtain the Magnet.
Note: This is the only magnet you can get in Pokémon Platinum, other than trading in another one from another game.
Magnet Details and Uses
The Magnet is a held item that boosts the power of Electric-type moves by 20% when given to a Pokémon.
As with most type-boosting moves, the Magnet is good for a lot of in-game battles—like fighting the ocean’s many Water-type Pokémon and trainers.
But there are much stronger items that aren’t limited to a single type.
For a couple of examples, look at the Life Orb and Choice Band/Choice Specs.
The Magnet is a safer option, if not quite as consequential in battles.
It’s arguably more useful than most other type-boosting moves, because:
- Electricity has fewer weaknesses than any other Pokémon type
- But they tend to be offensively underpowered
- The Magnet can balance them out and make your Electric-type more capable in most battles.