You should be able to get the ducting at any home improvement store, as far as the centrifugal fan goes any hydro store but your best price would be online from ebay or google shopping. I also have my air cooled hood open ended and i use it as an exhaust but i pump my air threw a carbon filter so a centrifugal fan is a requirement although any time i am dealing with an HID i consider a centrifugal fan a requirement i consider booster fans to be a liability.
What about odor arent you going to need to clean your exhaust air?