Just about any 100k thermistor on Amazon. Overall it’s still not an easy process. You’ll have to splice the wires or get new terminal pins like these:
And crimp them on and remove the old terminal pins from the wiring harness.
In addition to that you probably won’t be able to get the old thermistor out of the heater block, it’s just very difficult due to the stresses involved with the high temperatures and cooling. It may be easier to replace the heater block and start new. Then you may or may not get the heating cartridge out. Fair chance you’ll accidentally break those wires too and need to replace the heating element.
Sorry to sound negative but this is the reality of it. Not impossible but not a walk in the park either. Good luck!