WiFi Thermostat, No C Terminal on Furnace
Wirenut A is where you should tap C at

Since you have air conditioning, we can pull a little trick here.  We start with the yellow Y wire from the thermostat, noting that it is connected to one wire going off to the outdoor unit (condenser) via wirenut E.  Since we know that the thermostat is putting 24VAC from R onto Y to call for compressor, we know that the other wire coming back from the outdoor unit must be a C wire to complete the circuit.  Following it, we come to wirenut A, which just so happens to be quite a convenient place to make the thermostat C-wire connection at.