You can only find them naturally in one place, and thats the mages guild quest which you actually learn to make them. Other than that, you need to make them.
Noir77--- it's the offering for the vaermira quest.
Only at Dark Fissure "worm" at 1AM on "Shade of the Revanant" days (every 8 days) places one inside the alter (I believe).
SOME necromancer Adepts have them, but it's rare (not worth grinding for it). Best thing to do is join the mages guild and take all the grand soul gems from there and convert them.