The Necronom-i-nom drop from Necro Tome Treasure of Maldraxxus. To get this treasure you need to start a sidequest Read Between the Lines. Questgiver is located in the area a little bit to the north west of the treasure. Then after a series of quest you will get to the tower. And the treasure is on the top floor.
Here you can buy The Necronom-i-nom Toy Boost for EU or US Servers.
You'll get guide for completing The Necronom-i-nom Toy Boost