A good way to get Orichalcums is to find the Bulky Vendors. They go in a loop from planet to planet. Look in these planets to see which one you first find a Bulky Vendor in:

Agrabah ... in the Bazaar
Halloween Town ... Yuletide Hill
Beast Castle ... West Hall
The Land of Dragons ... Checkpoint
Olympus Coliseum ... Entrance to Cave of The Dead

Start with the planet you first found a Bulky Vendor and go down this list. Once it ends go back to the top. Orichalcums can help you make the Ultima Weapon. This system also helps you get eperiance points because Bulky Vendors give you 1100 exp. each! You might not get Orchalcums on the first try. But keep doing this and you will get Orchalcums!