A home thermostat will not work. I would just buy the thermostat online. If you order from a reputable site, what is not to trust? I am pretty sure petsmart sells the reptitemp 500r online. In the meantime you can go to home depot or lowes and get a piggy back light dimmer so you have at least some control over the heat pad.
Stick the probe thermometer under the substrate right over where the heating pad is. You want to measure the hottest point your snake can get to.