I ordered 20 of those piezos: http://www.puiaudio.com/product-detail.aspx?categoryId=5&partnumber=AB1541B-LW100-R. I got those particular ones for two reasons: PUI Audio is usually pretty good quality, and they are small (15mm diameter).

Here's a picture of most of them:

They are exactly 15mm wide, which is perfect. Another consequence of their small size is that their output voltage is lower than bigger sensors. That's going to be useful in the future, but I'll explain why in another post. They are also pretty cheap, as I got them on Mouser (ref: 665-AB1541B-LW100-R) for $22 ($1.10 a piece).

Well let's get to work now!