Rocket Warehouse
Rocket Warehouse, Kanto and the Sevii Islands (from Pokemon FireRed).
How to get here. Needs the Rainbow Pass, which comes from the Celio quest: finish the Tri-Pass islands, get the National Pokedex (it opens when you beat any Champion), bring Celio the Ruby from the Ruby Path cave in Mt. Ember, and he swaps your Tri-Pass for the Rainbow Pass. The Seagallop then sails to every Sevii island.
Items
On the ground (item balls)
Hidden (use the Itemfinder or press A on the spot)
Given by people