Where can I get a closed loop iPhone carrying case/holster?
The Ripoffs CO-IQ holster can be ordered with a belt loop (which is technically a BL-IQ using the RipOffs model number system). This is a low-profile, plain black nylon holster with a flap covering/securing the phone. At one time Ripoffs also made a CO-IP model for the iPhone, but it is no longer listed in their catalog. 



If you're looking for something more tactical, Maxpedition's 5" Clip-On Phone Holster can be adapted by using their TacTie Attachment straps as a belt loop. (You could also sew a nylon strap onto existing MOLLE loops to form a full-time belt loop.)



The 5.11 C5 Case-L can be snapped securely to a belt or attached to a MOLLE system. (The new style 5.11 tactical pants have a flap pocket especially designed for an iPhone that makes a belt case superfluous.)



Although I don't own any of these particular products, I've owned pouches, holsters and bags (and pants) from these manufacturers and am comfortable vouching for the overall quality of their products. 

Check your local Police/EMS supply shops if you want a local source. You may find other options there too—secure carry of smartphones is definitely something of interest to LEOS, EMTs, etc.