Which ultrasonic sensors available here will work with EZ-Robot? I have tried more than 8 different ultrasonic sensors from amazon.com and none of them have worked. There is some incompatibility in the inexpensive sensors with EZ-Robot. 
Unfortunately that leaves me buying them from the EZ-Robot company at $18 each plus $25 shipping.
@ThomasMesserschmidt Notice that the rear of the ultrasonic sensor has additional circuitry:
This changes it to a 3-wire setup. Therefore the cheap ones you purchased are correct, but lacking the extra circuit.
To the best of my knowledge, that small circuit is custom to EZ Robot.