Possible Ultrasonic Sensors:

http://www.automationdirect.com/static/specs/proxselection.pdf

Specifically the TU type:

http://web1.automationdirect.com/adc/Overview/Catalog/Sensors_-z-_Encoders/Ultrasonic_Sensors/30mm_Round_(TU_Series)

Another source of many possible ultrasonic sensors:

http://ultrasonic-transducers.globalspec.com/Industrial-Directory/large_transducer_ultrasonic