Thermal switches are mainly used for Rube-Goldberg type events such as Mission Possible.

Description

Thermal switches are broadly classified into resettable devices (trigger and then, after they have returned to the normal temperature, reset) and one-shot devices (act as fuses: they work once and cannot be reset). You can probably buy a bag of thermal fuses for the cost of a single resettable thermal switch, but the ressetable switch requires no maintenance or replacement. Like other switches, thermal switches can be found in Normally Open and Normally Closed configurations. They also respond to different temperature ranges, so picking one appropriate for what you can do in your machine is important.

Acquisition

Thermal fuses

These can be found in almost any appliance with a heating element (toasters, hair dryers, and the like). Since they are one-shot devices, however, the only reasonable way to acquire them is purchasing them, rather than salvaging them.

Resettable Thermal switches

These cannot be reliably found in small appliances. One manufacturer may use them, while another may use a thermal fuse. Purchasing is about your only option, but they are not expensive. As of this writing, the following surplus retailers carry these:

Search for "thermal switch". Expect to spend $1-$2, no more. Buy ones with know specifications (NC or NO, switching temp) if possible.