Most chain pet stores, especially Petsmartwill not carry thermostats. Yes you need one. Here is Hydrofarm, it's cheap and reliable. Here are Herpstats, much more expensive but they are great. And just for good measure here is Johnsons, I have one of these and it works very well for the price.
If you thermometer probe isn't even inside the enclosure, how do you measure temps inside?