I guess that's how Mitsubishi sells their cars cheaper, by making parts more expensive.
Perhaps you may be able to just unplug all the connectors from the heater and insulate them well to keep water and dirt from getting in them, then try starting the car again. The heater is located above an undercarriage shield just ahead of the battery. There shouldn't be any high voltage present as long as the car is off and unplugged. For added safety, you should disconnect the 12 volt battery.
If the heater has an internal short, it may cause an over current fault on its high voltage circuit, so the car doesn't start (heater, AC compressor, and motor inverter all get high voltage through one pair of contactors ).
As for a cheaper place to get a heater, the only option may be to get one from a wrecked car.
"Bear" - 2012 Diamond White Pearl ES with QC - 2/21/2013
"Koorz" - 2012 Cool Silver Metallic ES with QC - 1/5/2015
"Photon" - 2017 Bolt EV LT in Orange with QC - 7/31/2017
Solar-powered since 10/10/2013