Unfortunately, Route 12 is the only place in X/Y when it was available. If you really need some, trade, I guess? Or hack, but I don't recommend that :P
You could get as many Leftovers you want in the game by using the abillity Pickup. Pickup basically picks up items after you have battled. And if you have a Pokemon in level 80-100 you're Pickup Pokemon has a rare chance of finding leftovers. I would recommend you to level for example six bunnelby's to level 91+ and go fight random Pokemon.