I need a Mears electric heat thermostat, Model# M21DP. Where can I find one? I live in area code 54911.

You might find one that will work on eBay. If not do a Google search.

There is nothing special about a Mears, just need a 2 pole 240 volt line voltage thermostat.

Here is one:

Lux - Mechanical Thermostats: LV2-1042

If you need to match existing for aesthetics, then:
Line Voltage