As I understand it, after the Age of Sorcery ended, the Camps to turn in Sorcerers and their skulls have been removed (I am not sure if there is some Story element that explains what happened).
So, currently there is nowhere that you can turn in the Sorcerer skulls.

If you have any Obolus coins, you can use those at some NPCs that are still at Mekmosses’ Spire.

I hope that helps.