You may want to wait a little longer. Apple will announce iPhone 5S on September 10th but it looks like they may not start selling it right away. According to
this news article on Digitimes, Apple (TSMC) is experiencing yield issues with finger print sensor chip. Quote: "
Production of Apple's next iPhone, commonly referred to be the iPhone 5S and is scheduled to be unveiled on September 10, could reach only 3-4 million units in the third quarter of 2013 compared to 10 million units as originally planned due to a delay in production of fingerprint sensors needed for the iPhone 5S"