checkmateman

Best answer as chosen by Asker / Ratings
The main way to get Grand Soul Gems is through the many random loot crates scattered throughout the many dungeons over the map. The thing is, the loot containers generate what is known as "leveled loot", which means the higher your characters level, the better the loot. A level of about 20+ should be high enough for you to be seeing grand soul gems in such crates. Other then that there may be some stores that you can buy them in? Not too sure, maybe try checking all magic/enchanting stores.