They don't have the good one on the website, but I found it by all the other thermometers at my local Wal-mart. It was $12. Just ask an employee to direct you to the indoor/outdoor thermometers or you will never find them lol.

It looks like this:



Has a wire for the probe to go on the hotside
And a hygrometer

One thing I have noticed though is that if you use a heat lamp, the humidity can be off if its closed to the thermometer.