Re: measurements, I'd be very wary of the frequency response of anything you can produce with audio equipment that's not specifically designed for ultrasonics. The cheapest thing I've found for testing if you're getting some amount of ultrasonic signal up to 100kHz is a short spray from a can of computer dusting gas, but it varies hugely depending on a number of factors including how much gas is left in the can. Lately I've been using one of those pesticide-spraying bottles with a manual pump on it and running it dry with only air inside — this allows me to produce a long hiss of the same pressure for multiple tests.
?
Of course this also produces an unknown frequency response, but by shooting-out against a "flat" acoustics measurement mic and a CO-100k, I can use the calibration data of each mic to get a decent-enough profile of the frequency response of my noise source. This is the best I can do for the moment, but it's accurate enough for ballpark figures, so that when I do eventually pay for fancy sound lab measurements it will be giving me detailed data I can use for small tweaks in my design.
?
In terms of electret capsules, the best I've found so far is the Primo em258, but this has a pretty steep upper frequency rolloff, so your SNR at 100kHz is more than 40dB worse than it is at 20kHz, but even a Simple P48 build with this is going to give you access to frequencies that no regular studio mics will.
Zander