There are three classes of ultrasonic transducers that are usually found: piezoelectric, electrostatic, and PVDF. These three can not only serve as transducers, but can play the role of receiver as well.

PIEZOELECTRIC
 
For Example....

Nominal frequency: 40 kHz
SPL: 120dB
Min Detectable Range: 0.2m
Max Detectable Range: 6m

 
Notes:
    hard to array
    cheapest
 

ELECTROSTATIC

Nominal Frequency: 50 KHz
SPL: 115dB
Minimum receiving sensitivity: -42dB
 

Design Diagram

 
 
Size vs. Frequency

Notes:
    easy to array
    relatively expensive
 

PVDF

I did not investigate PVDF much at this time. I've been told it probably isn't the perfect stuff for this, so I'll come back to this at a later date.