Does it exist: 360 degree ultrasonic receiver/beacon combo
I have found an omni-drectional transmitter/receiver.

http://www.metrolog.net/transdutores/piezofilm/ultra40k.php?lang=en

More info and approximate pricing can be found on page 22 of this pdf: http://physics.ucsd.edu/neurophysics/Manuals/Measurement%20Specialities,%20Inc/Piezo%20Film%20Sensors%20Product%20Guide%20and%20Price%20List.pdf

There may be other/better sensors too, so I will wait a while to mark this as the answer.