Mummies can be found in World Adventures tombs. I don't know how to go about getting one in your household but I know it can be done.
To get a Mummy of your own, you need to find and assemble the 5 pieces of the Sarcophagus of Kings, and collect all 5 types of the canopic jars while raiding tombs. Then, with all the jars in a sim's inventory, that sim can summon a playable Mummy, complete with their very own Mummy name!
But there is another way to get a Mummy. There are two different varieties of Sarcophagus of Kings: Blessed and Cursed. If you have the Cursed version and have a sim sleep in it twice in a row for about 24 hours total, they will become a Mummy. Sleeping in the Blessed Sarcophagus of Kings will change a Mummy back into a normal sim.
Summoned Mummies are always elders, but they live a
very long time in spite of this. Sims can be turned into Mummies as early as their teenage years.
An interesting thing I read about a while back on Sims Wiki is that a Mummy will sleep in a sarcophagus indefinitely if instructed to do so. Thus, an interesting way to "preserve sims for future use" would be to turn them into a Mummy and then having them sleep in the Blessed Sarcophagus of Kings until you needed them. When you finally cancel the action, they will emerge from the sarcophagus completely unmummified.