How to power Honeywell wifi thermostat with external transformer
There should already be a transformer on the unit. I believe it's on the back of the unit, mounted to a junction box. There should be two exposed terminals on the transformer. One is R, and the other is C. To determine which is which, use a volt/multimeter to measure AC volts between the R wire going to the thermostat and the transformer terminals.  One terminal will measure 24 volts, while the other will measure 0 volts.  The terminal that gave you 24 VAC, is the C terminal.  Run a wire from that terminal, to the thermostat C terminal.