I wish they had these, when I had a wood-heated, off-grid cabin.
You may have one again...........whether you like it or not..........
They do, and they have been around for centuries. I think the term is thermopile and people used to power their radios with them before they were wired for electric.
Current term would be thermo electric couple. They keep your house from blowing up if the pilot light goes out on your water heater.
You can buy fans powered by the heat of your stove I looked at buying one a couple of years ago.