What vendors sell Exclusive Fortify Intelligence potions?

Andil, in the Tel Vos Service tower has one.

Other than that, they are just in leveled lists... so you will find them randomly through the world, sometimes appearing in the inventory of Merchants (usually Alchemists and Apothecaries.)

Five can also be found in a chest in the Velas Manor in Mournhold. You need to do the http://www.uesp.net/wiki/Tribunal:The_Summoner to gain access.

Why, you'll find it in your Mortar and Pestle, my good man! It appears right after you crush the Bloat and Ash Yam in there! If the first one is not exclusive enough for you, just drink it, then try again. Repeat until satisfied. You'll find the results astounding.

Yes yes good response, hats off to you sir.