I have an cheap inertial one that plugs into the cigar lighter socket. These sound much better.
http://view.harborfreightemail.com/...850c8cf180298410711a62de2fac5e477e3b04c1607d2
http://view.harborfreightemail.com/...850c8cf180298410711a62de2fac5e477e3b04c1607d2