Water to air heat exchangers can be purchased on the Internet from eBay and Alternative Heating & Supplies. The price usually depends on the size of the heat exchanger.
As of March 2015, eBay has an inventory of 12- by 12-inch water to air heat exchangers for around $70 along with 18- by 18-inch heat exchangers for around $130. Alternative Heating & Supplies sorts its water to air heat exchangers by BTU output and size. As the size and the BTU output of the heat exchangers rises, the price increases. For instance, a 60,000 BTU heat exchanger is around $90 and a 120,000 BTU heat exchanger is around $180.