You can get the Magic Missile Spell/Grenade Mod by defeating Wizards, Necromancers, or Handsome Sorcerer. If you are going to farm one, seek out Badass Wizards as they can drop a Purple/Rare variant. These are an x4 variant instead of a blue x2 variant. You can farm Badass Wizards in the ‘Temple of Misworship’ which is located in the ‘Lair of infinite agony’.