The "yellow area" in the fft looks to be centered around 13khz, so not really ultrasonic. Any audio grade transducer should work. Also since it is audio, you can probably use your PC sound card as the audio source and your PC speaker amps as the amplifier. I'm not sure right off what their amplitude measurement means (20dbB/1.00 m/s^2) but I doubt it is extremely loud.
As I was falling asleep, it occured to me that I might be able to use my TENS unit as a ready to roll driver for it, but then I remembered that its highest freq. was still low enough that I could sense the "vibes", indicating that it's much to low a freq, for this sort of application. The sound card idea is pretty good, it might be a good prototype test bed to find the best freq's, warble-rates, volume levels, etc. Once I home in on what works, I think I can rig up a pocket-sized box with ten bucks (or less) worth of stuff in it to drive the transducers.