There is no replaceable "chip" used today. The ECM can either be programmed through the OBD diagnostic connector using the proper equipment or the ECM can be removed & programmed using a PC with the proper software.
Several companies offer custom tuning by sending your ECM to them or by taking the car to them. Some may be site sponsors that could respond to this thread.
the ecm is normally kept in the engine bay, although im not sure where as i havent looking into the engine bay yet. there is no chip, just a solid ecm/pcm that can be removed for sending into tune. no chips