How do you get a purging stone dark souls?

Answer 1

Purging Stones are sold by;

Oswald of Carim (in the belltower after you defeat the Gargoyles in the Undead Parish, he appears after you ring the bell)

'Undead Merchant Female' (access to her starts by using the Basement Key to open the way to the aqueduct that you have already partially travelled through when you first entered the Undead Burg. She is at the opposite end to that part.)

You can place a Cracked Red Eye Orb in Snuggy the Crow's nest, and when you return, it will have been replaced by two Purging Stones.

Lastly they are a drop item from Man-Eating Shells.

Answer 2

You can buy purging stones in Dark Souls from Oswald of Carim in Undead Parish or from the Female Undead Merchant in the Lower Undead Burg. They are also sometimes dropped by Basilisks in the Depths or cursed frogs in the Darkroot Garden.