Weapon Upgrades (Elden Ring)

Knocked out the staff in the Lukaria Academy from the boss. Carian royal scepter. When upgrading, it requires not ordinary blacksmith stones, but stones of darkness.
Can anyone tell me where you can find stones of darkness +6 grade? And then I came across only up to level 5 and somewhere I got a piece of 8 lvl. I can’t find where 6-8 grades are pouring in.