outside boilers are just that, do a google search for local sources, they boil/heat liquid that is then pumped into the building just like a conventional boiler hot water radiant heat system using old fashioned ast iron radiators or the new version is pex tubing in an insulated concrete slab or under a conventional floor that is is insulated underneath and/or has a reflective barrier to direct the heat into the living space see radiantech online for more info.
I insulated under and installed tubing in the concrete basement floor of my energystar house when i built it last year, it's not hooked up yet but its there for if and when gas gets much more expensive.
Boilers are available here in Mi that burn wood, logs up to 4', wood pellets, corn, corncobs
etc, the pellet burners can be had with a self feeding auger setup you fill the hopper and the auger/stoker feeds the fire on demand, you can also get a gas/propane standby burner setup so that you don't have be home to feed the fire all the time
It's still most efficient to make the building air tight and well insulated first.