Is there a particular reason your parents are so opposed to getting a thermostat? Is it because they had a specific budget in mind for your ball python or because they don't believe you when you say a thermostat is important?
If it helps you could point out getting a thermostat will be cheaper than potential vet bills.
Have you shared your research with your parents are have they been pretty hands off? Either way good luck on convincing your parents! Maybe if they consider it as an early present for (insert upcoming occasion) they'd be okay with it?
Good luck!