You don't need anything too expensive. Just one that is reasonably accurate and uses a probe that you can secure in place. A couple of these would probably do the trick https://www.amazon.com/dp/B0017N8W90/ I'd mount one on the basking surface and one on the cool side surface so you can see the gradient and basking temp at a glance. It's also helpful to have an IR temp gun to spot check temps in and out of the tank https://www.amazon.com/Nubee-Temperature-Non-contact-Infrared-Thermometer/dp/B00CVHIJDK