How do you get leftovers in Pokemon SoulSilver?

Answer 1

On route 12, after you wake up the snorlax, it is holding one. You can catch it or use thief. You you use thief to steal them, then flee and go defeat the elite four again, and return to route 12 the snorlax will be there again, allowing you to rinse and repeat.

Alternatively, after you collect all 16 badges, the elite four gets new teams, and on koga's team, he has a swalot holding a leftovers. If you use skill swap (to get rid of the swalot's stick hold, that prevents thief from stealing items) then you use theif, then you can steal leftovers once per run of elite four this way. IrecommendAlakazam for this, because it can learn both moves required to do this (skill swap, you get this TM for beating sabrina in kanto, and thief, which you also get from playing the game) and it still has good moves.