At Petco they have ZooMed digital probe thermometers for about 8 bucks. We have two and they seem to work fine. My snake spends a lot of time on the cold side too, although he uses the warm side after he's eaten to digest. I think that's because his warm hide was too exposed, but I've fixed that and now he goes back and forth between the two sides. So, make sure both of your hides are nice and closed in, maybe with just a small hole for entry. You definitely want a thermostat; they have fairly inexpensive ones at Petco also.