You don't even need to buy an amplifier. You can stick the key against your head and the water in your brain will amplify the signal. Bonus points for taking a plastic container of water for even further extension of the range!
http://physics.stackexchange.com/questions/101913/why-does-a...
https://www.youtube.com/watch?v=0Uqf71muwWc
EDIT: i_cannot_hack got the link in before me. Kudos ;)