[cabmandone]

Yes you can buy a propane conversion kit for a furnace but it'll be through a licensed dealer and the dealer will have to install it. Dealers won't take the liability risk of allowing a homeowner to convert. When converting you have to change the burner orifices, gas valve spring, and add a low pressure switch to the inlet side of the valve. You then have to get the gas pressure set which requires a gauge or manometer to read gas pressure.

I can't speak to water heaters because they sell those as LP or NG. But every furnace I've ever sold came from the factory as NG and was converted to LPG where necessary. Every stove I've ever owned since living in my house needed to be converted and included a kit inside the stove. Every clothes dryer... same thing.