I bought a simple temp gun online from ProExotics and it works fine for me; the cheapest model is about 25 dollars. But I feel that you should probably get a thermometer with probes, because I doubt you'd want to be tempgunning the tanks 24/7 to check the temps, and you never know when something will happen and heating units go on the fritz and fry your snake!
I got mine from Reptile Basics and I use it all the time. I also use a thermometer/hygrometer so that I can monitor the ambient temps and the humidity.
~~McKinsey~~
"Men have forgotten this truth," said the fox. "But you must not forget it. You become responsible, forever, for what you have tamed."
~The Little Prince; Antoine de Saint Exupery