How can I use a separate transformer to provide a C wire to my thermostat?
You guys down voted the only answer that is correct and up voted one that is completely wrong!!! You CANNOT use just one wire from a transformer and expect to get it to do a thing for you. The simple answer to your question is that if you add a transformer you want to tie the lines from it to Rh and C. Most modern thermostats I know of expect to pull power between Rh and C. You do NOT want to tie Rh and Rc together since your heating and cooling are on separate transformers and you don't know if they are truly floating.

The complex answer is that you should check to see if the control board of your heating system doesn't, in fact, have a common terminal and nobody bothered to run the wires for it. If it does, it might be simpler to just pull 18/3 wire from the heater to the thermostat. Depending on the run, you can use the old two conductor wire to pull the three conductor wire through.