Well this probably depends on your level, as most loot and merchant items in this game are leveled...
Check the Mages guild alchemists, they should have some grand soul gems for sale at 200 septims a pop. I had to go to three different mages guild merchants to get 5 soul gems... and now I cant find Nightshade leaves
Some other mechants also sell soul gems, usually the general merchandise merchants.
And if you are sneaky and good with a lockpick, the mages guild in Chorrol has some Grand Soul Gems in the display cases.
i found a grand soul gem in layawiin in the mages gold .well u have to buy it, i also killed a shop keeper for one, still searching for 2 more, !!!, any one no were to get 3 more soul gem stones
bought it in some store in the imperial city for like 300 something gold.... dont remember which store... and didnt even buy it for the vampirism as i didnt even know i had it at the time (was looking to do some enchanting)... till i noticed some red smoke coming off me while in the arena... so now im a stinkin vampire... so i have to find 5 more grand soul gems... then go visit this witch lady at the combolo river.. and she will tell me what to do from there RIGHT.... Been playing 14 hours straight now and i'm not stoppin till i get of the vampire garbage... Kinda like the Good guy role in the game. WAIT.. just read that you could get ride of the vampirism by praying at a shrine... is that true?
This site is part of the Defy Media Gaming network
The best serving of video game culture, since 2001. Whether you're looking for news, reviews, walkthroughs, or the biggest collection of PC gaming files on the planet, Game Front has you covered. We also make no illusions about gaming: it's supposed to be fun. Browse gaming galleries, humor lists, and honest, short-form reporting. Game on!