Hi all,
I know Jim's link went to an S-type Pitot tube, it will work, but requires a much larger hole and also has a measurement factor associated with it. It is often used for environmental stack testing and very large ducts.
This is the option I would recommend http://www.dwyer-inst.com/PDF_files/122d.pdf. The 166/167-12 is very handy and will accommodate up to a 12" duct. You only need a 3/16" hole in the duct for insertion. The model with the shorter tip (1" I think) would probably be the easiest to use on smaller duct.