How do you fill a black soul gem?

In Skyrim you can either take them from a person you have killed or more commonly they are found as random loot, usually in caves where a lot of necromancers live. They can also be crafted with the lightning attractor in the Soul Cairn (Dawnguard DLC), can be found from geode veins in Blackreach and can be purchased from Enthir in Winterhold and Falion in Morthal.