The easiest way to get the mummitonium is to buy it. Go to the relic merchant each day and check his inventory. You may have to try several times. If he has only one piece, you could try posting it for smelting, you may be lucky and get two ingots back.
While the quest is current, the mummitonium will be marked in town view, if there is any available.
I once found an ingot in a chest in one of the small tombs, but, this is very rare.
There are other ways, but if you are in Egypt these are the ways to obtain it there.