Single pole 120 v thermostat
StackExchange is not for product recommendations.   However, any of the simple, mechanical $13 "mains wiring" 240V type thermostats should suffice, and a 1-pole unit will suffice.   (On 2-pole, only one one pole is switched thermostatically, the other is simply an off switch.   They mount in a standard junction box.  

Do not use an incandescent bulb when you want a heater, it is cruel: incandescents burn out randomly and then the poor creature suffers until you notice to replace it. Use a heater.  Catalogs like McMaster-Carr and Grainger sell lots and lots of heating components.   

You can either use a 120V heater of your desired wattage, or a 240V heater of four times the wattage.  For instance if you want 50W and you see a heater that is perfect except for being 240V and 200W, that will do exactly what you want: draw 50W if you connect it to 120V.