to get regular orichalcums you can obtain them from the 5 chests there are, or you have a 10% chance of getting one out of a bulky vendor
bulky vendors are extremely rare heartless that randomly show up, and can only be beaten with reaction commands
and to show how *RARE* they are, i played the game on standard and only got one bulky vendor beating the whole game, and that was in the market street area of Agrabah on the 2nd visit