The Pololu accelerometer would likely work under the right conditions- a stomp ought to produce a sharp, short duration acceleration. What would concern me is that if these are stones in dirt, the motion might be pretty damped, so your signal might be small. I'm guessing, though. It might be fine.
I was interested in similar signals for a different reason, and I used piezoelectric coax- it looks like thin coaxial cable, but the insulating part inside is PVDF. Stepping on this yields a big voltage, tiny current. You can get it from a company called measurement specialties, but it isn't all that convenient to buy it from them. You can find the cable on eBay, where it is sold as inexpensive pickups for amplifying guitars.
There is a nice document on how this is used in an application closer to what you are doing, with some circuit diagrams.http://www.docstoc.com/docs/8894270/PIEZO-CABLE