Well, I have listed some rarenot obvious items (Pokemon hold items, randomly found items, etc.) here in this cheat.
Black Sludge-Wild Croagunk and Toxicroak can be holding these when battling them.
Quick Claw-A person in Jubilife City will give this to you.
King's Rock-Wild Poliwhirl hold there.
Dragon Fang-Wild Bagon may be holding these sometimes.
Amulet Coin-You can find this hidden in one of the small buildings in Amity Square.
Leftovers-Found in the Victory Road. (The part that is blocked off by someone until you get the National Dex)
Life Orb-An area in Stark Mountain.
Master Ball-(Besides getting it from Cyrus)You can get this in the Lottery Corner in Jubilife City. It is the top prize and the lotto number has to exactly match your Trainer Card.