Katina, it is difficult to know what you are wanting, but I will assume it is an overvoltage protection device.
The cheapest devices are varistors, also called Voltage Dependent Resistors (VDRs) or Metal oxide Varistors (MOVs) or Transient Voltage Suppressors (TVSs). The Wikipedia article here gives you an idea of the types and how they work. These are bi-directional devices usually used for protecting AC circuits from overvoltages (lightning etc).
They are available in a huge variety of sizes, voltages and protection currents. The ones I am used to - like the top photograph in the Wikipedia article - are like small disks, usually brightly coloured (though the colour does not mean anything) and are often mistaken for capacitors. You can get higher capacity ones that bolt onto a ground such as a switchboard enclosure.For DC applications there are the devices such as the P6KE series. They are like high power Zener diodes and are available in voltages from a few volts up to several hundred volts. They would be used for protecting DC power lines from overvoltages. Bidirectional types for AC are also available.I hope this has answered your question?Cheers / David
