Most of the rpm measurement/speedometer projects use a Hall sensor rather than a reed switch, because is no mechanical switch bounce to deal with and they are quite fast.
I have successfully used the Hamlin flange mount 55140 or 55100 series in some projects. A good source for strong magnets of all shapes and sizes is: http://www.kjmagnetics.com/
You will need to mount the magnet so the polarity is correct for the Hall sensor. Some Hall sensors are activated by facing a south pole, and other by facing a north pole.