There was a thread about this topic recently, and for the life of me, I can't find it. >.<
Anyways, I had a similar experience where I really wanted to keep a Sim's urn, but it was nowhere to be found. Sims would go to mourn in front of the bed, as far as I could tell. This was my method to get the ghost and the urn, if you really, really don't want to use mods...
Unfortunately, there's no easy way to summon a ghost if a writer didn't make a book about them before they died. I usually just move everyone out of the ghost's lot except a vampire, because they can hibernate in a coffin indefinitely. Let time speed up and if it slows down suddenly for reasons other than a phone call, odds are the ghost is in the house somewhere. If you have City Living, the ghost might also invite you to a festival. However, you cannot invite them into the household during a social event. In this case, save when you arrive at the festival, then cancel the social event and quickly ask the ghost to hang out. (They tend to vanish right when the social event ends, so the save lets you try again.)
You need to have the friendship bar nearly completely full for the "Invite to Household" option to come up. When the ghost is in your household, the urn cannot be destroyed. So you can demolish the lot the urn is on and it will automatically go to the household inventory. I don't know if there's other ways of making the urn incapable of being destroyed, but if there are, it would be useful to know.
