how to find a gravestone?

Graveyards usually have a team of people responsible for site management. I'm pretty sure they also keep a record of whose was buried in each plot. If you can find anyone on site, or contact them in some way, they might be able to tell you. I doubt it's especially confidential, since the names are carved on the monuments.

It's probably worth a try. I don't know if there will always be someone available at the site, though. When you think about it, though, someone has to be responsible for the site, or it would be rather chaotic finding an appropriate place to inter the deceased.

Jam it back in, in the dark.