Grave of the False King has 1 or 2 slaves that you can find. It is on the hill, that is near the starting area where you find the fisherman.
Hollow Tunnels has 2 or 3 slaves that you can find and free. It is located on the far southern end of the map, close to the coast line.
The Iron Creed fortress has 3 or 4 slaves that you can free. The slaves there get a little buggy and they will occasionally be found attacking each other (or you). Just spam E until you can talk to one.
Maiden's Last rest has 2 or 3 slaves that you can find and free.
Mira, at the town of Graycott can be charmed akin to how you tame monsters to be your companion. Masturbate near her or use the Alluring staff to have sex with her, after 3 times she should become your companion/slave.
That dungeon you mentioned actually does have 2 or 3 slaves that you can free. They should be down the left passage (and probably the same down the right hand passage as well). Once you kill all of the monsters in the final room at the end of either passage, there should be prison bars lining the back and side walls of that room and inside those prison cells there will be slaves that you can free.
They are intentionally hidden in the shadows to make them easier to miss.
Here is a screenshot of my map, it should have MOST of the locations for dungeons, ruins, and various other places. There is always the chance that this map does not have everything possible in the game. I don't run around looking for every single thing I can find, just most of everything.
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