www.Contact Microphones.com
   If you need contact mics you might just get a piezo element and solder it to a cheap wire and connect the desired output right? Bingo, contact mic ready to go.  Then before you know it the cheap wire you used and bad connection begins to fall apart or the wire is to short to really do anything fun.  Before you know it you have to get the soldering gun out and that thing becomes a terrible piece of d.i.y. crap. 
    If you want to go the easy route and buy professionally built contact mics(the best solution)  Lianne's mics are the best.  They feature high quality AV cable, great outputs that you can get in any size and stereo capabilites.  I got the double 1/4" stereo set and have never been happier.  They sound awesome through my 4-channel pre-amp.
   She also builds some really interesting hydrophones that are entirely waterproof to record underwater sounds.  For more information on these contact microphones go to
www.contactmicrophones.com

Update: New contact mics, medical grade suction cup mics,  emi sheilded film tab mics, internal pickups, and optimins now available at Liane's site.  check them out. I just bought a suction cup mic and will post a review once I get to use it for some recordings.  I can't wait to record things through glass, I'll also be posting some samples of the drum pictures at the bottom of the page soon on http://fieldrecordings.def-recs.com
SUCTION CUP MIC
NEW
INTERNAL PICKUP!
NEW
BUFFERED PHANTOM POWER CONTACT MIC