Hey there,
A thermostat is a important component of any Ball Pythons habitat, that's great you are looking to upgrade.
I recommend a proportional thermostat. You can read about the difference between on/off and proportional thermostats here:http://www.reptilebasics.com/thermostat_faq
My personal recommendation would be Reptile Basics Vivarium series of thermostats: http://www.reptilebasics.com/thermostats
or
Herpstats - I own a Herpstat and a Herpstat ND and I love them both. It doesn't get much better than Herpstat:http://spyderrobotics.com/index.html