If there are other household members left in the household, and if the sim died on their home lot
you might be lucky enough to find their urn if you'll go play with them soon after.
but if no one's left in that household, or the sim died off lot there's no way to get the urn without a mod.
LittleMsSam's have a mod for retrieving urns of any ghosts in your game, as long as their spirit wasn't released yet
you can find it here under the name Release all Ghosts & Get Urn for
https://littlemssam.tumblr.com/post...-are-sooo-small