Not sure what your budget is, but some of the cheaper ones that many people use are:
Jumpstart:
https://www.amazon.com/MTPRTC-ETL-Ce...art+thermostat
Inkbird:
https://www.amazon.com/Inkbird-Tempe...ile+thermostat
If you want to get fancy and have more money to spend, a HerpStat is a great choice:
http://www.spyderrobotics.com/index....=index&cPath=1
The Herpstat will offer higher accuracy and is a proportional thermostat. This means that it will gradually adjust the power to your heat source in order to maintain a near constant temperature. The other two will turn off completely when you reach the desired temperature. When your temperature drops about 2° they will turn back on until your heat source reaches temperature again.
When you do get your thermostat, make sure you place the thermostat probe between the heat pad and your tank. DO NOT place the thermostat probe inside your tank as your snake can move or bump it resulting in inconsistent heating.