You can get a rheostat dimmer from the petstore. Almost all petstores have this product:
Although with that you'd have to manually control the heat. I'd rather just spend the money right now for a proper thermostat. Herpstats, vivarium electronics, helix...etc they are all good thermostats.
Stick on thermometers are no good. Get something like an accurite or even a zoomed/exoterra thermometer with a probe. Infrared heat guns are also a great investment, as you can point and shoot to get a reading anytime you want.
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