1.) Their's a guy on Route 2 that wants to see some Pokemon (Cherrim, Magcargo, and Zebstrika) and he'll give you a Magnet along with a Charcoal, Miracle Seed, and the TM for Nature Power.
2.) Wild Nosepass have a 5% chance to be holding one (found in Caratos Mountain.)
3.) If you've finished v9, you can buy them in GDC's Underground.
4.) Their's a Magnet hidden in West Gearen (I think it's outside the east side of the Power Plant.)