Choosing the right liquid flow ultrasonic transducer

I am looking for an ultrasonic transducer (just the sensor) for a liquid flow meter i plan to build but i am stuck at choosing the right one for the job. i found these sensors from Audiowell (http://www.audiowell.com/en/product-detail.aspx?id=250) but i believe they are built for invasive application. I would like to build a non invasive system, but can't seem to find the right sensor. All i can find on google are complete flow meters. I am guessing they will have to be stronger so that its output can penetrate whatever pipe it is attached to. Any ideas on where i should be looking or what else i should be considering are welcomed.
thanks