It's not obvious that you need a thermostat. What you need is an indication of the temperature in your system and the object that Mick described, which is in fact a thermometer rather than a thermostat can be popped on by anybody. You need two, one for flow and one for return.
If your return temperature turns out to be undesirably low you can do something about it then. For a wood burning boiler maybe 10 deg difference would be fine. The usual adjustment is by means of a bypass with a gate valve.
I've never heard of a thermostat in the main circulation, although TRVs are commonplace on radiators, and it sounds like a seriously bad idea to me.