I think it is useful to point out:
A square wave drive through a series tuned circuit is fine because a series tuned LC circuit presents a high impedance to the harmonics contained in the square wave.
Whereas a parallel tuned circuit presents a -low- impedance to the harmonics, causing losses in the drive transistors.
A bare piezoelectric ultrasonic transducer's equivalent circuit is a capacitor in parallel with a series-tuned LCR circuit. With just half-an-H-bridge square wave drive, the transistors must charge and discharge that parallel equivalent capacitor on each half cycle.
That equivalent circuit as described above is going to have two resonances: series tuned and parallel tuned resonance. Ideally, I think you'll want the low impedance series tuned resonance.
I've never bought from this company, but $8.50 each seems a lot less expensive than $25 each:
I'm doing a lot of Google searches for ultrasonic cleaner design. My head is spinning. The lower power designs mostly just get away with overrating the components and driving with a raw square wave.
I must add that an ultrasonic cleaner is potentially hazardous. Probably not those crappy plastic "jewelry cleaners", but the higher powered ones. Note that this video suggests a test that results in a layer of aluminum foil being destroyed. Now imagine cavitation going on inside your hand if you reach into a running ultrasonic cleaner.