From the http://www.uesp.net/wiki/Oblivion:Black_Soul_Gem:
Starting at level 5 there is a 10% chance that any Necromancer Adept (necromancer boss) will be carrying a Black Soul Gem in their inventory. A list of locations containing Necromancer Adepts is provided at Necromancer Dungeons. The Necromancers in Howling Cave for Meridia's Daedric quest also have a 10% chance of carrying Black Soul Gems.
The only guaranteed place to find a couple black soul gems is in Falcar's room in the Chedinhal Mages guildhall, but they aren't real gems.
That leaves hunting necromancer bosses, of which there aren't many. I've killed LOTS of necromancers in my time, and only found a couple gems. There's also the ones in Howling Cave, but IIRC those don't respawn.
It's really much much easier to just make your own gems, but if you're deadset against doing that, be ready to kill lots and lots and lots of necromancers.