The person who posted this image originally posted it on imgur in 2013 with the description:

The poster is really nice too.

I'm weighing it down with some freebie cowbells from State Farm (...?) and fans from freecreditscore.com. Comic Con is a wondrous thing.

Which means it came from Comic Con.

The actual artwork used on the poster is the same artwork used on the PS3 Ni No Kuni All-in-one Edition from japan. At least the top portion of the artwork.

The chance of getting the poster is nearly impossible as it is from a Con. Even getting the officially released PS3 limited content cost hundreds of dollars since they sold more than they created, and screwed a ton of people out of the Wizards edition canceling orders. I have the physical copy of the spellbook sitting in a collectors cabinet, and it is still my all time favorite collectors item from any game. A pretty much exact and full recreation of the book from in the game.