Board Thread:Skyrim/@comment-107.140.186.207-20150320004302/@comment-6881629-20150320104459

There are lots of ways and places to get a black soul gem check the wiki page here but you might get one from Gulum-Ei in that guild quest if your high enough level, you have a chance of mining them from geode veins in Blackreach if your in the collage you can buy them from Enthir etc.