If its the white all round light on the back you can buy them direct from fletcher pick the 17 foot boat at
http://www.fletcher-boats.co.uk and then parts list:
F17 024 Navigation Lights 42" Pole Light - White (each) £28.50 £33.49
I fixed mine but was looking around the net and many online places sell them as well what make is yours? Mine was an Aqua Signal 20 Series Anchor light like this one
http://www.boatingstore.com/ search for "AQUA SIGNAL 20 INCLINED POLE LIGHTS"
Have a look on the net to see if you can get one cheaper
__________________