Long story short, it’s in the top right corner. I used an app called Proximity Sensor Finder just to make sure of its location.
A lot of people seem to have problems with the power button after a few months use, as do I. If you are similarly afflicted, the best solution I’ve come across is Proximity Actions, which allows you to use wave gestures or holding of the sensor (indicated above) to turn on and off your phone. I’ve messed around with it a bit, and the one-second hold option seems to be the most reliable and satisfying.