Here are all the locations where you can get them in Elden Ring. Although you might already know, it’s important to note that Somber Smithing Stone is used for special weapons (armaments).

Now's the time to get all the bell bearings to ... On your left will be Smithing Master Iji. Iji offers you a unlimited Somber Smithing Stone [1] and [2], plus a finite number of Somber Smithing ...