I'm having troubles finding the invisibility spell tome at all. I have Illusion at level 90 and I went to the illusion trainer in the College of Winterhold and he wasn't selling it either. I have checked all over the internet and I coudn't find anything else on how to get it, so where am I supposed to find it? I'd like to know where to buy it as it seems nobody is selling it.
The reason I'm asking is that I'm playing as a Dark Elf illusion thief, but the twist is I can't use weapons so I need invisibility.
Best Answer
According to the UESP (A source I prefer to the various wikias), the only guaranteed source for the Invisibility spell is Drevis Neloren, who will sell it to the player once their Illusion skill is 65+.
It is possible that if your Illusion skill has just reached that threshold, Drevis will not have it in stock. If this appears to be the case, simply trigger a merchant restock (by waiting 2 days), and he should have every Expert Illusion spell (of which, Invisibility is one).
Beyond that, the console commands in Trent's answer will give you immediate results.