You can have a look at the HaroUT library.
However, typically you need a digitizer that would acquire at least twice as fast as the signal you want to acquire.
I suppose that you intend to oversample but you will be dependent on the stability of your signal.
In general, ultrasonic signals are acquired using fast digitizer like NI Oscilloscope series. An alternative is to use a scope and transfer the data over USB, Network, or GPIB.
The HaroUT library has a built-in hardware driver for the Olympus (now Evident) FocusPX, that can digitizer multiple channels up to 100 MS/s (it is a $20-60 k instrument). It also has a simulator that you can use to learn how to use the signal processing.
Marc Dubois