They're a donut shaped piezo, I tried to source these from a Google search, but found not one. But I repaired an old one by glueing 4 10 mm peizos around the cymbal hole and wiring them in parallel. But I had to put a resistor in line on the phase connection to reduce the signal as I came out quite hot! If you're using a drumit3 (or the soon to be drumit5 mk2) , this might not be necessary as it does have a gain reduction option in settings..
But if you contact 2box, maybe you can source where they got their donut peizos from.
