Steering angle sensors work by having one central gear driving two gears that rotate at slightly different speeds (one gear has one more tooth than the other gear). The gears have magnets at their centres. Two magnetic (Hall effect) sensors then detect the position of each gear as a sinusoidal wave over 360 degrees. Processing circuitary uses the phase difference between the sensors to calculate the absolute position of the central gear, even after several complete revolutions (with each successive revolution, the phase difference becomes greater).
Processing circuitry on the sensor returns a digital value to the ABS computer, so will hopefully be easy to interface to your system. Your system needs to accept an arbitrary value as the 'zero' point, since the output of the sensor will be non-zero at the midpoint of the steering travel.
Any car with ESP/VDC (Bosch ABS 5.7) will have a steering angle sensor, such as a 2002 or later Alfa Romeo 156 or 147/GT that includes the sensor shown below. However, the sensor is probably stupidly expensive to buy as a replacement part.
Perhaps try a Hyundai agent, as I understand the relatively-cheap Hyundai Getz has stability control as standard on all models (at least, in New Zealand it does). Hyundai used to include lots of Bosch parts, so it is probably going to be a standard item similar to that used by Alfa Romeo.
-Alex
