The Sphere of Silence is an item found in the Cave West of Aubout du Monde in chapter 4 of Dragon Quest IV which casts Fizzle. It is found on the same floor as Oojam, yet another lift must be taken to get to it. Oojam himself gives hints about its whereabouts.
It must be taken into consideration that the Sphere of Silence does not always work and its main purpose is to make the battle with Balzack easier for the party with it being rendered nearly useless after the end of the chapter.
Also, in Dragon Quest VII (3DS), it is a useable item in battle which might stop an enemy from casting any spells.