MaxxAirWe use the MaxxAir cover. They are pretty much the standard that most dealers stock. But it's pretty basic in design so I'd not get hung up on the brand.
You also might take a look at
MaxxAir vent fan for better ventilation. The fan moves much more air that that little 4" fan supplied in many bathrooms. Some models are auto on/open and off/close so you can set a temperature and it will cycle on and off to maintain temperature. Some have remote controls since the fans are not always in a place you can reach. Some can exhaust and intake (change flow direction).
Just depends on your budget and what your trying to accomplish.