It is not a hard thing to do and you hardly need anything.
For some people this is something obviously simple but for someone like me these things seems out of reach and difficult, but reading this and realizing that it does not have to be complicated makes me happy.
For this the parts could be had for less than five moneys...
You need a spring, a small speaker and a touch microphone.
A small speaker is easy to source from an old toy (or Ebay, just checked and saw some for one euro each plus postage...) and the touch microphones are very cheap.
After reading this I did order some through Ebay and it cost me less than ten bucks for ten of them. Half of that for postage! Always wanted some of those in either case as they seem to be a lot of fun to play around with.
The article/tutorial comes with some other neat tricks to.
Recommended Sunday reading!