New bathroom vent fan, old bathroom!
Shut the power off at the breaker and remove the fan. (If you have a Volt Meter to check the lines at the fan, I'd do that for insurance.) Bring it w/you to either a Home Center or a Plumbing Supply House (found in the YELLOW PAGES under PLUMBING SUPPLIES.)
I don't suspect your fan was installed from 'above', no? If so, if you have an attic or crawlspace up there, then that's where you need to go to remove it. (I hope the 'exhaust' is being sent outside via an insulated duct.)