Mission Possible/Thermal Devices

Acquisition
Small electric cooling pads (talking about active cooling here, not just fans) may be using a Peltier device. Beverage heaters/warmers usually use resistive heaters, so those won't help you.

As of this writing, these surplus retailers offered Peltier devices. Just search for "peltier" or "thermoelectric".
 * Electronics Goldmine
 * American Sci & Surplus
 * All Electronics

Most other surplus electronics retailers probably carry them too. Expect to pay $10 to $15 dollars for a small one.

Peltier device
See above section.

Bimetallic strip
See Bimetallic strip for more information.

Thermistor
Thermistors are resistors whose resistance is closely tied to the ambient temperature. All resistors respond to temperature changes, but thermistors are designed specifically to respond to temperature changes. Thermistors are extremely inexpensive ways to detect changes in temperature. If the current rules allow digital electronics, thermistors are very versitile and can be put in circuits capable of reacting to small changes in temperature. In years without digital electronics, thermistors are less ideal, but still an option; large changes in temperature will have to be used to affect a large enough change in resistance to be useful.

Acquisition
All surplus electronics retailers should carry thermistors. Expect to pay $1 for a package of one or more. Look for the cool and hot resistances to determine whether they would be useful in your application.