WalMart would be your best bet for a RELIABLE and ACCURATE thermometer/hygrometer combo unit. Those stick-on things from pet stores are, well, crap, and are usually off by more degrees than I want to acknowledge.

Look for the Acu-Rite Digital Thermometer/Hygrometer Combo unit w/Probe. They cost around $11-$15 and can be found in your local WalMart in the hardware/indoor-outdoor thermometer section. Look behind the paint section and where the electrical stuff is.

When you get the unit, place the unit itself flat on the floor of the cool side, next to the hide, and run the probe over and up under the warm side hide. This will measure your cool side floor temps, your warm side hide temps and your humidity. Or you could be anal and get two units and place them like this:


You place both units about halfway up on the wall of the enclosure, one on each end. Then run the respective probes of the units under the respective hides on each end of the enclosure. This way, you get ambient temps, hide temps, and relative humidity for each side(warm and cool).

Hope that helped