If you bought the extruder kit from here, then the thermistor would be included. I highly doubt they would have this at radio shack honestly. The extruder assembly will have two sets of wires, one for the heater and the other is the thermistor. You will probably have to source a replacement online. I do not have this extruder and I can’t say for sure which one you would need. Maybe somebody else will comment on an exact replacement for this extruder.