My furnace is wired with a 2 wire thermostat. I need to add/run a common (c) wire to my new wifi thermostat (currently running off batteries).

The easy part is finding the common, the hard part is getting a wire to the stat. What is the make and model number on the furnace?

See how many conductors are in the T'stat wire. If only 2, you will need to run a 3. If you are lucky, you may be able to tie the two together and pull back, lots of time the low voltage wires are not nailed onto the studs.

Silly question but why not just run it on batteries. I have 4 thermostats & they all run on batteries which I need to change ever 2-3 years. Nearly all digital thermostats have built in freeze protection which, in case the batteries go dead, will hold the heat at 40-45*.