No sensor off a non-Triumph make will fit--at least not without modification.
The only place to find a new sensor is
www.bikebandit.com or through a Triumph dealership. They are not cheap, and they run about $180 if I'm not mistaken. You can sometimes find them on eBay or other classifieds, but I don't recommend buying a used one, as the magnets break every few years, so it would be a shame to spend the money on a used one just to replace it a few months later.