Acoustic Guitar pickups are pretty good for this, they are cheap and sturdy. If you go with the kind that the piezo is the actual saddle, they are about 1/8" thick, you can cut and shape them and they can be cut up iinto multipul sensors. Soldering on them is abit of a trick, but not to bad. There are also the type that goes under a guitars saddle, its like foam wrapped in tin foil, and tends to have a deeper sound if you hook them up to an audio source. if you dig around the net abit you can find them for very cheap, or if you know someone in the acoustic guitar repair field. I find they work better then the disc type and hold up to abuse much better.
adam