You can buy (or otherwise get) soul gems in Oblivion mainly from magic shops, Mages Guild halls, and loot on mages/necromancers.

Quick Scoop: Where to Buy Soul Gems in Oblivion

1. Easiest places to buy soul gems

These are your go‑to shops if you just want to walk in and spend gold:

  • Mystic Emporium – Imperial City Market District
    • Sells multiple soul gems, including grand ones, both empty and filled.
* Two merchants inside (Calindil and Aurelinwae), each with their own stock, so you can sometimes grab several in one visit.
  • The Gilded Carafe – Imperial City Market District
    • General magic/alchemy shop that can stock soul gems among other supplies.
  • Other magic shops in Imperial City Market District
    • Players report buying and stealing soul gems from shops “next to Slash and Smash” and Rindir’s staffs store displays.

2. Mages Guild halls: buy, loot, or “borrow”

Once you join the Mages Guild, guild halls become the most reliable way to stock up:

  • Every Mages Guild hall has an alchemist or magical supplier
    • These merchants can sell soul gems of various sizes (including grand), with inventory that changes over time and with your level.
  • Chorrol, Skingrad, Bravil, Anvil Mages Guilds
    • Frequently mentioned as good spots to:
      • Buy soul gems from the resident mage/alchemist.
  * Pick locks on display cases and grab extra gems if you don’t mind crime.
  • Arcane University lobby
    • Has several grand soul gems sitting in the lobby area; they can be taken or stolen depending on your status and sneaking skill.

Many players in forum discussions recommend “join the Mages Guild, then raid the halls” as the most efficient way to build up a pile of soul gems.

3. If you specifically need grand or empty grand soul gems

Grand and especially empty grand soul gems are often tied to the cure vampirism quest and high‑end enchanting.

  • Buying empty grand soul gems
    • Mystic Emporium in the Imperial City: merchants can sell empty grand soul gems (watch the description carefully so you don’t buy a filled one by mistake).
* Various Mages Guild halls (notably Chorrol, Skingrad, Bravil, Anvil) may have them in merchant stock or display cases.
  • Scripted quest locations (cure for vampirism)
    • After talking to Melisande in Drakelowe, secret areas in Squandered Mine and Crayfish Cave open up, each containing empty grand soul gems you can pick up.
* These are intended to help you complete the “five empty grand soul gems” requirement for the cure.
  • Loot and world finds
    • Many necromancer and mage dungeons, boss chests, and high‑level loot lists can spawn grand soul gems as you level.

4. Other ways players get lots of soul gems

Forum discussions and guides mention a few “meta” methods players commonly turn to:

  • Duplication glitch
    • Widely suggested as a “last resort” way to multiply soul gems if you don’t mind exploiting glitches.
  • Turning grand soul gems into black soul gems
    • If you want powerful souls, you can use necromancer altars at the right time of night to convert regular grand soul gems into black soul gems and trap humanoid souls.
  • Killing/looting mages and necromancers
    • Many carry soul gems; repeatedly clearing out necromancer lairs is a steady, if slower, source.

5. Mini checklist: “I need soul gems fast”

If you just want a quick path:

  1. Join the Mages Guild and check every guild hall’s alchemist/merchant.
  1. Visit Mystic Emporium and The Gilded Carafe in the Imperial City Market District.
  1. Loot or pickpocket soul gems from Chorrol / Skingrad / Bravil / Anvil guild halls and the Arcane University lobby.
  1. If you’re on the cure vampirism quest, clear Squandered Mine and Crayfish Cave for guaranteed empty grand soul gems.
  1. If still short and you don’t mind exploits, use the duplication glitch on one empty grand soul gem.

Information gathered from public forums or data available on the internet and portrayed here.