Melody,

The controller you want is called a humidistat. You can buy it at most
Heat and AC stores but these stores may not sell retail. I'm not sure that
your plan will work however. First, why do you think that humidity is
causing problems? Have you measured the humidity? A cheap hygometer can be
bought at Radio Shack for about $30. I would think that an average humidity
above 70% would be needed to cause problems and you should be seeing mold
and mildew as well. Second, Your plan is the replace the air in your house
with air from outside so measure the humidity outside. Third, figure out
where the source of the humidity is and try and fix the problem. Finally, if
all else fails go with your fan, though AC or a dehumidifier would be much
better. You'll need a whole house fan such as a large attic fan wired to the
humidistat rather than a bathroom vent fan.
All in all I think that one fan would be fairly expensive and also noisy.

Good luck,
Dave M.