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Jamie,
      I've used cheap Radio  Shack peizo elements ($1.25) as contact mics. 
 
Run the sound through something that can act like a pre-amp (many effects 
processors, for instance) or a real pre-amp/amp and you're in business.  
There are nickel-sized ones and quarter-sized ones as I recall.  I buy 'em 
by 
the dozen. ;-)
Good sounds,
      Bill "Hawkeye"