Before laying out cash make sure it doesn't just need to be adjusted (it was bumped too far back into the tube) or it is not simply dirty. If you really do need one, contact the manufacturer. Kisan, for example, sells replacement sensors for $10-12, but they use three different ones so you need to know the tyoe you have (two-pin, three-pin, or dual Molex).
Kisan ones are sold at this site.
Safety Electronics For Motorcycles
I just put mine on a toggle switch, so I can turn it on or off as I please.