As I see it, you've got a couple options - either the StuG Batterie from Grey Wolf, or the StuG Abteilung from Earth & Steel. Grey Wolf is the Eastern Front Late War Book, while E&S is the Invasion of Normany book.
I'd suggest you head over to
Easy Army and pick up either one (or both!) of the books, as they're very cheap.
Then head on over to the Plastic Soldier Company for their
StuG Company Deal which will get you 15 StuG III G's at a fraction of the price of Battlefront's tanks!