Gold Ore can be bought from Clint for 400G or found in the mines. It is harvested by a Pickaxe from rocks past level 80. A good method of farming it can be to use the elevator and get to floor 80. Then go to 81 until you see gold ore appear. Typically it will show up there, and continuing to floor 85 from 81 provides a nice amount of gold ore.
Five Gold Ore can be smelted in a furnace to create a Gold Bar