Add C Wire with transformer

Good Evening I want to put in a wireless thermostat but dont have a c wire. I was reading online that i could use a additional thermostat since all 5 wires are in use. I have a 2 stage cool, 1 stage heat system. With R connected to R/Rh (Jumper) If i get a 24V Transformer and hook 1 wire to C where does the other one go. Here is a picture of my setup: Image: http://postimg.org/image/uiuv63aet/