I want to get more leftovers and was wondering if there are any trainers who have Pokemon who have leftovers so I can use theif to take them away. I know that there is the pickup method but it takes too long and was hoping this would be an easier way if there are any trainers like this. Thanks in advance.
Remember, Thief's new mechanic from generation 5 still applies to X&Y - if you steal a trainer's item, it will be returned at the end of the battle. So knowing these trainers, if any, is absolutely useless. And no wild Pokemon hold Leftovers in X&Y, so Pickup or Powersaves are your only shot in getting Leftovers.
The simple and sad answer is no. You'd think the Battle Maison would have trainers with leftovers or at least have a trainer in the building who gives you the item, but no.