There is no simple "Potion of Invisibility" per se in The Elder Scrolls V: Skyrim, but there are several with different durations. They are:
- Potion of Brief Invisibility
- Potion of Enduring Invisibility
- Potion of Extended Invisibility
- Potion of Prolonged Invisibility
Or you can make your own using ingredients with the Invisibility effect or the following recipe:
Shadr, a stable hand in Riften also gives the Dragonborn one of these potions as reward for clearing his debt.