Honestly, I'd be using an under tank heater with a thermostat. They really need belly heat. Plus, in the long run, it will be cheaper than buying bulbs every few months for 10+ years. But if you are dead set against an UTH with a thermostat ($250 for a UTH and a nice thermostat that you only need to buy once) and will only use a heat lamp ($10 bulbs every 3-4 months for 10+ years...you do the math), I'd just go buy the bulb. Your snake needs some form of heat and 77 is just too low for digestion. Go ahead and order 1-2 of the bulbs online and buy a bulb from the petstore. Then, when that bulb burns out, you already have a spare bulb at home to replace it with. I used heat lamps for a long time for my corn snakes before I got a rack for them and that's what I did.