The Armor Polish can first be acquired in Nobilia Marketplace by trading the Silver Sheath and 75 Jewels. The player can trade the Armor Polish back for the Sheath for another 75 Jewels at the same merchant, and may repeat this as they wish. Keep in mind though that the Silver Sheath is glitched in does nothing.
In Ivor Tower Alley, if the player traded away or sold the Armor Polish previously, they can buy it back in the alley for 300 Gold Coins. The alley closes when the player defeats Mungola, so the purchase must be made beforehand.