The only cheaper way I can think of is to make your own ultrasonic transducers from raw piezo elements (these are available - or at least used to be - because a long time ago I used them for tuning the frequency of lasers). You would need to machine a tapered metal block to mount them on, in order to match their stiffness to the compressibility of water. And you would need to waterproof them.
Or, with help from Google, you may be able yo find some less expensive transducers designed for use underwater. Remember, it's not just that you need waterproof transducers, you also need a reasonable acoustic impedance match between the transducer and the water it is immersed in.