There are thermostats on Amazon in the $20-25 range. I find it strange that Petco/PetSmart would consider that too expensive when there are bags of specialty dog food that cost twice that amount.
Selling a thermostat with a heating appliance is a stupidly easy money-making upsell for a retailer, and I'm rather shocked that the stores don't do it.
At any rate, unless/until OP returns to state 1) how she is measuring temperatures, and 2) whether or not her heat sources were regulated, we don't know if she cooked her snake due to ignorance, or if the store sold her a sick one.