here are a couple of sources for piezo discs for contact microphones, vibration/knock sensors, underwater contact microphones, and more:
Electronic Goldmine as piezo discs with and without plastic casing.
Digi-Key sells several diameters of piezo discs.
This page is an archive of recent entries in the piezos category.
equipment access is the previous category.
server is the next category.
Find recent content on the main index or look in the archives to find all content.