in leaf green you cant find ekans, arbok, oddish, gloom, vileplume, psyduck, golduck, growlithe, arcanine, shellder, cloyster, scyther, and electobuzz but you can trade them from fire red.
Piplup does not exist in Pokemon LeafGreen or FireRed.
You cannot catch houndour in Pokemon leafgreen either get one from Pokemon emerald or Pokemon colosseum.
That item doesn't exist in leafgreen.
Nowhere.
on a island