I know that there's a certain artifact that stores infinite amounts of Black Souls, but I don't wish to defy Lady Azura. I'm going to be using the Bound Battleaxe to imprison innocent souls, all I need is a good supply of Black Souls Gems.

They're just so evil! I love them so much, but I can never find them. Does anyone know of a good place to get Black Soul Gems? For some reason I only ever use white souls, which is too nice. I want to be evil.

The soul cairn is my first thought.

Another is to buy it from falion in morthal though I'm not sure if he only has them for the cure quest.

Most of the time, it's random.

Thanks for advice, but are there any locations in vanilla Skyrim that I can find Black Soul Gems?

Or should I make a trip to Solstheim?

Hagravens sometimes have them so you could check with them.

Also, conjuration mages in caves...though there isn't anywhere specific...

Falion in Morthal sells them, sometimes he even has two in stock.

If you're with the College, Enthir usually has one for sale at a ridiculous price.

You can also randomly find them in Blackreach by mining the Geodes there. Otherwise you can find them as random loot once you're at a certain level.