I'm sure if you were looking for a particular model, I or someone could help you more directly. But in the absence of that, I respectfully suggest you talk to someone in a Plumbing Supply House. (Look in the YELLOW PAGES under PLUMBING SUPPLIES.) Once you get a make and model, you can search one out on your own.
Make sure it will do the job. There are some 'minimums' regarding cubic feet per hour in terms of moving air through the fan. Be sure you don't 'dump' the air into the attic or any other unheated 'space'. If your ductwork must be installed THROUGH an unheated space, make sure it's insulated. If it isn't, the moisture will condense and run back into the bathroom, and cause rusting and other problems, like mushy drywall.