Where can I find black soul gems?
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?
Solution 1:
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.
Solution 2:
You can also find them in Geode Clusters in Blackreach. It isn't a merchant, but it is mineable. Note that you'll also get other things: corundum, normal soul gems, and occasionally gems.