Here is a link to one of the best priced thermostats available. I've used this product since October and it works fine for me. Other purpose built thermostats can get very expensive. You would plug the heating pad into the thermostat, and put the probe that comes out from it between the heating pad and the glass. It will measure the heat coming out of it. You simply set it to whatever temp needed for the temp in the terrarium to be 90-92 degrees. You have have to set it up between 95-100 degrees to attain proper temps inside the terrarium because some heat is lost through the glass and any substrate. This will now keep temps regulated and keep your animal safe.
http://www.amazon.com/Hydrofarm-MTPR.../dp/B000NZZG3S