The Necronomicon is an ancient book of dark, necromantic magic by Abdul Alhazred and is the first book about the Elder Gods such as Cthulhu. It is nearly impossible to get an authentic copy of it today, since the dark powers described within the Necronomicon are so powerful that most people who try to find the Necronomicon die grisly deaths before their obsession with finding it is ever fulfilled.
If anyone were to find the Necronomicon and somehow survive, they would go completely insane, but they would also gain omnipotent powers by chanting the black magic spells contained within, becoming a god themselves.
There are many FAKE Necronomicons you can buy online, but teh Internets don't actually have any REAL copies for sale. Even the FAKE Necronomicons have powerful enough dark magicks to overwhelm mere humans. Here is a video of someone sharing a (most likely fake) copy of it with someone else (if you watch it alone your eyes will explode (eye explosion not guaranteed. No refunds.)):
Here is a factual video explaining the true history of the Necronomicon: