Soul Coins are items in Baldur's Gate 3 that you can obtain through all sorts of means: exploring, opening chests, rewards from quests or saving random people, etc.
Their use are still unsure but you can sell them for quite a bit of gold. We'll update once we know the actual use for these.
- Coordinates: -295, -325
Inside the southern-most room of the Dank Crypt where all the sarcophagus are. Check inside the northwestern one.
- Coordinates: -321, -271
Inside the Dank Crypt, from the large room with the statue of a god, enter the southwestern room to find a sarcophagus that has the coin in it.
- Coordinates: -293, -229
Inside the Dank Crypt, it's inside the sarcophagus where you found/met Withers.
- Coordinates: 243, 448
You get it from the Tiefling Nadira who's at the top of the southeastern hill in the Druids' Grove. You can get it as a rewards from her if you pass the [Persuasion] check and actually save her from the nearby Bugbear. If the Bugbear happens to kill her, you can loot it from her body.