Try www.airshack.com. They have nice, quiet Broan units that have the
humidity sensor. When the wall switch is in the on position it turns
on automatically when someone is using the shower, and then shuts
itself off automatically after a few minutes. By flicking the wall
switch off then on you can turn the fan on manually as well.

You can adjust the humidity sensitivity for the sensor and the shut off
time on the unit. Uses standard wall switches and costs about $125 if
I remember correctly.

The ones I found at the big box stores were waaaay too loud for my
tastes.