I've missed the chance to get the artifact that acts as a permanent black soul gem so I'm looking for them elsewhere.
Which merchants regularly sells black soul gems I can use?
Find black soul gems
the-elder-scrolls-v-skyrim
Related Solutions
The Cronvangr Cave in Eastmarch is full of weak spiders, I usually hunt there.
Map, courtesy of the Elder Scrolls Wikia:
Black Soul Gems are exactly like Grand Soul Gems - with one significant perk:
They can trap humanoid souls.
Regular soul gems are restricted to creatures and undead only.
But black soul gems?
Tired of the blacksmith sending out hits on you? Kill him, trap him in a black soul gem, enchant him into a ring, and then sell the ring back to his wife. Justice is served!
Best Answer
In the College of Winterhold's living quarters, you can find a black market merchant named Enthir - he'll sell you Black Soul Gems for something like 1200 or 1300 septims.