There's one in the Desert Palace where Kuja lives! To get the second one, go to Memoria and proceed with the game until you reach a room that looks as if it's under water! You'll see a big stone(or sometihng like it) on the right! Take a right turn in front of the stone and walk downwards in the parts where you can't see yourself(behind the weeds) and keep hitting the X button! Someone will ask you to leave and you can either choose to leave or to stay there! Choose to stay and you will fight Haydes! After the @#$% difficult fight, you'll get the second Pumice Piece! After that, there's supposed to be something you have to look for in Memoria to get the Arc! I don't really know about this so ask around!