I know that the RAA100 uses resistive rudder angle sensors; who manufactures resistive sensors?

The RAA100 is compatible with resistive rudder style sending unit. The brand, VDO manufacturers a sensor (Figure#1) that can output 10-180Ω of resistance range. Simply connect the RAA100’s white and black wires to the VDO sender’s positive(White wire) and negative(Black wire) connection to convert the sensor’s resistive value to an NMEA2000 rudder angle indication message.  

figure 1) part number: 440003

Visit our RAA100 example system for in depth screen shots of a rudder angle on a DSM250 display.

https://www.maretron.com/examples/products/raa100/RAA100BasicSystem.php