Here it can be bought at every major photographer's "etc" store... one of those stores carrying GPS units, night visors, weather stations, telescopes, and yes, thermometers. I never understood what this equipment's got to do with photography, but hey, they sell it.
The glass shapes are all slightly different weights and their buoyancy means that they rise or sink depending on the density of the water that decreases as the temperature rises. Each one usually has a metal plate attached to the base with a number showing its temperature. My parents have got one.