You can build a temperature controller circuit with the 555 IC together with a thermistor resistor divider. The advantage is that a well regulated power supply is not needed. The dividing network consists of adjustable resistor R3, thermistor R4 and R5.
DANGER: Electric Shock Risk! This circuit is connected to the main 220V line, therefore it must be handle only by experts!
how does the temperature controller circuit works?
When the thermistor temperature is below a set value the voltage at pin 2 of the 555 drops below 1/3 of Vcc. This turns on the triac controlled heater and also starts the timing cycle.
If the thermistor temperature rises above the set point before the end of the timing cycle the heater shuts off at the end of the timing period. Otherwise the heater continues to stay on.
Thermistors at different values can be used as long as R3 + R4 = 2 * R5 holds true at the desired temperature.