I just purchased the Mophie Juice Pack Air for my 3 GS and have been trying to locate a vertical pouch case locally. Since I couldn't find one I thought maybe some on here would know of an oversized one.
May be you need something not special designed for iPhone 3G S, but in bigger size. I got one pouch from here, take a look if it also available to you. Good luck! Uxsight: Pouch Cases - iPhone 3G / 3GS: