I don't know if you can get a lower temperature sensor for auxilary fan...
however, you can get a lower thermostat. What seems to be the problem? It
sounds like your aux fan is not coming on when it is supposed to... and in
those situations, you simply need to change all the sensors.

Other thing you can do is backflush the radiator/condenser fins to clean it
of bugs and dirt... Use air hose or water from inside engine bay and shoot
out toward the front of the car.