Get a Pokemon with the ability Frisk (lets you see the opponents held item) or Compound Eyes (Increases accuracy, but also increases the chances of wild Pokemon having their held Item if that Pokemon is the lead Pokemon) and have it learn the move Theif (TM 46) or Covet. Than go to Virbank Complex and look for Magnemites. They have a 5% chance of their held Item being Metal Coat (7.5% if you go with the Compound Eye method). I recommend a Shuppet/Banette since they high attack making them use the move Thief effectively and can also have the ability Frisk. Just make sure that your Pokemon is not holding anything or the item stealing part of the move wont work
Happy Hunting
