DIY audio Spolight

[nanonash]

Hi,

I am trying to build a DIY AudioSpotlight (http://www.holosonics.com/ for an introduction)

So basically I need to find some good (and cheap) ultrasound transducers.. Has anyone got an idea where to get them ?

I would like to work in the 40-44 KHz frequency band..

There is the Pioneer PT-R9.. (goes up to 120KHz!) but it costs a fortune..

Thanks!