duct, duct fan and damper available here   http://www.iaqsource.com/product.php/fantech/fr100/?product=111037&category=1251     Next question is where are you going to run the duct?  - putting a new hole in the roof or side of house might be avoided if you can find an existing vent (house or bathroom) to connect into. 


If you want to get really fancy you could use a motion detector and a timer that would run the fan for set time after kitty pooops.