I am trying to find a soul shard for the succubus in Royal City, but I don't remember where I can obtain them.

I think there is a merchant in the slums sewars that sells them, the hooded guy at the table with the ninja in Rose's club.
If you wanted to collect them the only thing I've seen drop them is the ghosts in the manor, but they aren't farmable after you clear it that I've seen.