I actually saw one at Walmart yesterday. They also had the cheapy preg tests for .88. I was surprised.
I am going to go out on a limb and say that as long as you get a thermometer and stay with the same one, you should be ok. It is not actually the temp you are looking for, it is the fluctuation in the temperatures.
I could be completely wrong. But that is what I seem to be reading from everything!!
Best of luck!