In The Elder Scrolls IV: Oblivion (including Oblivion Remastered), you can buy the Invisibility spell primarily from Illusion trainers or Mages Guild vendors once your Illusion skill is sufficiently high, typically around level 50. Key locations to purchase the spell include:

• Delphine Jend at the Bravil Mages Guild
• Calindil at the Mystic Emporium in the Imperial City
• The Mages Guild in Bravil, southeast of the Imperial City, is a common place to find vendors selling invisibility spells

The invisibility spells available vary by duration (e.g., 30 seconds, 60 seconds, 120 seconds) and your Illusion skill level affects which spells you can buy and use

. If the vendor does not immediately offer the spell for sale, waiting a few in-game hours may refresh their inventory

. To use the Invisibility spell, you need a reasonably high Illusion skill (around 50 or more), and casting the spell will render you invisible temporarily until you perform an action like attacking or interacting with NPCs or objects

. For a related stealth spell, the Chameleon spell-which provides partial invisibility-can also be bought early in the game at the Bravil Mages Guild from the vendor Ita Rienus, but it requires some Illusion skill as well

. In summary, to buy an Invisibility spell in Oblivion, visit the Mages Guild in Bravil or the Mystic Emporium in the Imperial City after reaching about level 50 Illusion skill, and purchase it from Illusion spell vendors there