Thermostat wiring 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.

Don't use an external transformer. Find a way to run an extra wire from the furnace/air handler, and wire it as a proper C wire.