Where can I find Grand Soul Gems on Elder Scrolls IV: Oblivion

In order to cure my character's vampirism, I need to find 5 Grand Soul Gems to give to the witch. Where can I find these?

Solution 1:

The walkthrough for the Vampire Cure quest on the UESP Wiki gives locations. Mostly you have to find them while adventuring, but starting this quest places 3 of them in nearby dungeons which would have been previously unavailable prior to starting the Vampire Cure quest.

From the Wiki page, places that always have an empty Grand Soul Gem:

• Calindil at the Mystic Emporium in the Market District has one, which can be bought or pickpocketed.
• With the exception of Skingrad, the merchant in each city's Mages Guild has one which can either be bought or pickpocketed.
• You can steal one from Melisande's basement (accessible through the fireplace next to her bed).
• There is one in the Arch-Mage's Lobby of the Arcane University. It is in a locked display case along with two more Grand Soul Gems that are not empty.
• There is one in a locked display case in the Mage Quarters of the Arcane University at the bottom of the staircase on the left as you enter.
• The Chorrol Mages Guild has an empty one in a locked display case in a room just left of the front door, and another one in a display case beside the stairs. If you are a Guild member, picking the locks and taking the gems will not count as a crime.

Solution 2:

If you are not interested in the quest, you can get the Vile Lair Official Plug-in and use the font of renewal to cure your vamprism.