You could purchase on those hydrofarms or Big apples thermostat until you can afford a Herpstat OR even better go to a local reptil forum, here in Canada we have reptilescanada.com and ALOT of people sell extra thermostats, for example someone is selling 2 Vivarium electronics for 100$ each or $150 for both. Thats prolly the second best option, if you cant afford a new 1, look at buying a used (most still work always make sure they work before you buy)
And from here on out, try to save 10-20$ a week (skip lunch 2 days a week) and put in a piggybank in 2-3 months boom you can get yourself a nice brand new herpstat and your snake will be happy as ever