I have seen two different types of senders used: one is an optic sender and I think it was available from the old Skystar org. (but I don't know where in the open market to get it) and I think it required the you paint the header tank black so it would work.

The other is a float type sender. I used the float type sender and that is available from Aircraft Spruce. The part number is 6905-400 (Fuel Warning Sensor).

There could others as well so maybe someone else can provide additional input.