As of February 2015, discounted furnace parts are available from several online retailers such as Amazon, HVAC Parts Outlet and Midwest HVAC. Discounted furnace parts can also be found at salvaged building material retailers and some installation companies.
Online retailers tend to cater to most major brand manufacturers. Amazon can be a good place to hunt down discontinued and hard-to-find parts. It is also a good place to look for rebuilt and refurbished parts. HVAC Parts Outlet offers a large selection of new products at a wide range of prices and has a section dedicated entirely to furnace parts. Midwest HVAC sells all its products below the manufacturer's suggested retail price.