In Pokemon D/P/Pt, leftovers can be Held by wild Munchlax, found in Victory Road, and obtained with the ability Pickup.
In HGSS, Held by the wild Snorlax,obtained with Pickup ability, and one is held by Koga's Swalot (but I think it has sticky hold as an ability, so you have to do something about that.)
Leftovers can also be bought on Pokemon battle revolution for. 7200 Poké Coupons.