Does anyone know where I can find either the Thunder Monger stats, or the Gun Smith class for the Artificer? I have checked practically everywhere on player class book guides, and even in equipment book, but I can't seem to find it.
This specialization does not exist in 4E
The Gunsmith Artificer, complete with the Thunder Monger power is a brand new addition that was introduced in the D&D 5E's Unearthed Arcana playtest materials, found here.
Prior to 5E there is no official D&D (or Pathfinder, for that matter) content that paints an Artificer as a gun-wielder.
This was a sufficiently striking change from how Artificers worked in 3.5/4e that Keith Baker, the guy who invented the Eberron setting (which introduced Artificers to Dungeons and Dragons) posted on his blog to talk about how weird he thought it was to give the magi-scientist with a specialization in enchanting stuff for themselves and others...an actual gun.
If you're still looking to play D&D 4E, then I unfortunately have to tell you that the Gunsmith specialization does not exist in the edition you're playing. It was introduced in 5E.