How do I get the invisibility spell

[Izzet]

.

[DPMR]

You can buy from
http://en.uesp.net/wiki/Skyrim:Drevis_Neloren

[Mephitic]

The spell is less useful than invisibility potions, in my experience. Silent Casting is a must if you want it to be effective without detection. Potions are easily crafted and the effect is longer than the spell if you have high crafting skills and perks.

[Solomon Hawk]

Not a spell with the "Shadow Stone". A bit southeast of Riften near the Nightingale Sanctuary.

[DPMR]

So there you have it, the spell is allways useable, the potion till you run out of them and the Shadow Stone once a day.
Take your choise, or use all three.

[Olivinism]

Drevis Neloren sells this spell at random once the Dragonborn reaches level 75 in Illusion.
May rarely be found in random chests.
May be sold by random vendors.

[HiTmAn]

just get ilusion at 75 and check mage traders

[NecroMaster]

Here’s how spells in shops work.
All novice level spells can be purchased from mages and randomly in general stores at any point in the game.

Advanced and adept level spells can be purchased from mages when you reach the proper skill level which is 25 and 50

Expert level spells may only be purchased from the trainers at the college when you reach level 75 on the required skill

Master level spells may only be learned through quests available at level 90 and can be purchased afterward.

The only other way to acquire spells is random drops from mage enemy’s but seeing as how this is completely random and the odds of rare spells is slim this is unreliable.

There are other unique spells that can’t be obtained from mages such as the transmute ore in the halted stream camps