Some online websites, such as Aaron's Equipment and Surplus Record, list used furnaces and boilers for sale. However, the high cost of transporting central heating equipment means that shopping locally is generally the best way to find a used furnace at a reasonable price.
Local HVAC stores might carry used furnaces and boiler, although most save floor space for newer models. The high cost of furnaces and boilers means that they're typically used for as long as possible. Old furnaces and boilers are likely to operate inefficiently and might need expensive repair work. In addition, laws might forbid their installation due to safety and environmental issues.