There are a few online stores that sell underwater ultrasonic transducers, e.g. in the UK Maplin and Rapid sell this one Prowave 200LM450 Underwater ultrasonic transducer, 200KHz, plastic case | Rapid Online.
There are a few online stores that sell underwater ultrasonic transducers, e.g. in the UK Maplin and Rapid sell this one Prowave 200LM450 Underwater ultrasonic transducer, 200KHz, plastic case | Rapid Online.