check ebay, they are usually on there for like 5 bucks if you cant find a free one.
The exciter ring is the ring around the hub that almost looks like a big gear. Basically, the way your abs works is you have a sensor, basically a magnetic pickup, which sees the “teeth” on the exciter ring flying by it, and it detects the teeth and compares it to the other wheels speeds. If one is going faster than another (under braking) then the ABS will kick on untill all wheels are spinning the same speed.
If your sensor is not picking up the exciter ring, it will think that the sensor is not working, even though it is. So, check your exciter ring as well as the sensor.
How are the teeth supposed to look? Mine are all there, they are a little rusty but not bad.
Basically, if the sensor is not picking up the signal from the passing metal teeth, then you are screwed. Your best bet might be to hit it with a wire brush and clean it off as much as you can. If you have some spraypaint, you can hit it with a LIGHT coat just to hinder the rust.