Rheostats are a great emergency/interim measure if your house ambient temperatures don't swing, and they're readily available at big box hardware stores like Lowes or Home Depot (look for a "lamp dimmer"). They won't keep a heat source at a constant temp like a thermostat will but they will help prevent it from getting dangerously hot.
For a king snake I'd use either a deep heat projector (DHP) or ceramic heat emitter (CHE) instead of a bulb as you can set them and leave them on all the time; they don't emit light. They also last a lot longer than bulbs - think years not months - and all of the energy they draw becomes heat, so while they're more expensive initially they will save you money over time. The room ambient light is fine for a day/night light cycle.
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