An Inline Security Thermostat?

I have been searching around and to no avail. My first heater, a cheap one I will admit, stuck on me and was on its way to frying my first fish. Luckily I caught it and had a backup.

Is there such thing as an inline thermostat? One where you plug the heater into and a probe would go into the tank to determine the temperature. Just incase the heater fails and remains 'on' then the inline thermostat would sense the temperature going above a maximum threshold, one I set, and shut off the outlet until it decreases in temperature and then cycles it on and off to retain the temperature.

So I essentially need something like this:

http://www.amazon.com/Johnson-Controls-Digital-Thermostat-Control/dp/B00368D6JA/ref=pd_cp_hi_0

but cheaper, obviously?

I'll check out the Ranco too.

Alright. Thanks dubley and SubRosa. I'll get on the hunt soon.

Seems efficient. Price isn't too bad either. I will keep looking around though; I just need something simple to shut off the outlet if the temp goes above a set limit and maybe, if it's available, an alarm for if the temp goes below a set limit. I have got to be the most paranoid person...

That's what I need! Are they expensive? Is Apex the brand or a model name?