something where I can create and control ultrasonic frequencies
You need to define your frequency range.
Like loudspeakers, ultrasonic transducers tend to be tuned to specific frequency bands.
40kHz transducers can be had quite cheaply, though you'd have to take them apart to couple them to your probe.
You also didn't mention power levels - your probe could be a dental pick or smaller, or it could be a cattle prod.
The reason I mentioned a cleaner is because it would be a source of a cheap high power transducer and drive electronics.