heater/defroster problems

I have a 2000 Isuzu Rodeo 4x4 3.2 w/air. 110.000 miles. On winter trips to cold areas below 32 degrees the defroster and heater are unable to keep interior warm and the windows from foging up. I was thinking of changing the thermostat from the standard 170 to 195 but every part store and even dealer was unable to find one. First would there be damage to the sensor devices if I changed out the thermostat? And do you know where I can get a 195 for this model? Is there another answer to this problem besides blocking the radiator whith cardboard (an old trick).

Thanks, Carlos Flores