Where can I find a transformer that will step up a 12v input to around 400v?
Your driving frequency(from the oscilloscope plot) seems to be too low for ultrasound. The ATX PSU transformer is designed to work at more than 25khz and will have a low impedance at your frequency causing it to overload the power supply. This might explain the low Vp-p of the output waveform.

Since you are new to electronics, I wouldn't recommend building this circuit due to the risk of high voltage and damage from ultrasound.