Altimeters do not measure altitude directly. They measure atmoshperic pressure. An altimeter is realistically a barometer with a specific purpose. Alimeters use changes in atmospheric pressure to determine changes in altitude. To put it in simple terms, the pressure at any given moment in the atmosphere on earth is resulted from the weight of the atmospher above (pulled by gravity). Thus, the higher in the atmosphere, the less atmosphere you have above, and the less pressure is exerted onto you.
There are many types of altimiters from Digital, Analog, altimeter watches, and much more.
Here are some links to help you find some altimeters.
Froogle Search Froogle Search for Altimeter Watches
Froogle Altimeter Search