Do you want it to measure fermentation temperatures or mash temperatures. If the latter, you have to check the maximum temperature that the thermometer will read. After looking around for one to insert into my HLT and mash tun I found those aquarium ones and many of the home indoor/outdoor ones won't read greater than 45-50oC.
I'm considering these models: Y5031, Q1452. The first one is kind of cool because you can angle the head back up to 90o so good for sticking into mash tuns etc and through the walls of things.