I have been looking into this on and off again for a couple of years.
The gauges seem very easy to find and I have one of my tank. The sensor cable with the wire seems to be very rare and the ones that are already connected to something or expensive.
I found this video tonight and the company that posted it claims to be willing to sell the sensors.
I contacted sales at DPStele. Com. I will post here if I actually get a hold of one of these sensors.