The easiest way to find Leftovers in Black/White is at Village Bridge, hidden in a garbage can opposite the man in the blue shirt, south of the bridge (map). You may use your Dowsing Machine to find it.
You can also get Leftovers from an in-game trade in Undella Town, where you can give a Cinccino to a man in an orange shirt to get a Munchlax holding Leftovers. It also also possible to get Leftovers via Pickup for Pokemon over level 80.
