Why are 3-way center-off wall switched so rare / expensive?
Simply lay two common switches right next to each other.  One is SPST and turns the whole caboodle on and off.  The other is a plain 3-way which selects between "always" or "thermostatic" - common goes to the switched-hot off the SPST, one traveler goes straight to the fan, and the other traveler feeds the thermostat. 

You'll need /3 cable between the switches and thermostat, or between switches and fan if the thermostat is on a spur. 

If you only have a 1-gang space, you can get dual 3-way switches -- you bridge both commons to each other.  On the top switch, one of the "on/off" travelers is disused and the other goes to supply. 

The other option is to do the thermostat wiring in 24V low voltage, then you can use any switch you please.

Of course, if your AHJ (local inspector) approves switching mains power with an electronics-tier switch which lacks a UL listing or has a ЯU electronic component listing, then you can use it. NEC 110.2.