Where do you get the confringo spellbook in Harry Potter and Deathly Hallows game?

You actually don't get a Confringo spellbook in Harry Potter and the Deathly Hallows game.

The Confringo spell is learned through the "Dueling Club" side quest. It's part of the standard progression of spells you learn during the game, and you don't need a specific spellbook for it.

There are other spells that require spellbooks, like the "Stupefy" spell, which you get from "Professor Flitwick's Magical Creatures" side quest.

Hope this clarifies!