If you are still having this problem these are for sale on EBAY $73.00. RHD Japan stocks them also. Also search from any JDM site. They are out there.

I know this is an old thread, but I thought I would share my info for future searchers.
I have a 1977 280z,  (77/78 Senders mount on top). This is the Thermistor I used to fix my Fuel light. This at your own risk as the sensor resides in the fuel tank, wrong thermistors can overheat and be dangerous. I'm just sharing what I used. The light has been working for a year now.