Thanks for the question.
You said you cleaned the venting was the vent blocked at all
This thermal cut off thermostat is on the heater housing at the top pull the plug and join the wires together if the heat starts up replace the thermostat, or take the element out and look for a break in the coil.
Yes you can buy it at Sears or any appliance parts store in your area great job.
Thanks and God Bless.