Almost forgot, Roger that Tiny mentioned, does a very good job on the heaters he fixes up. He has sold many restored Chevrolet heaters on ebay under the seller name of "Casseltait". Plan on spending $200-$300 for a restored Chevrolet heater.
Otherwise a heater is an easy restoration project if you find a solid one to begin with that isn't all rusted out. Basically a heater consists of a 2-piece sheetmetal housing with vent doors on the front, a small heater core (radiator) for the hot water to run through, and a small round blower motor to spin the fan blade and blow the air across the heater core. The blower motor gets wired to a switch under the dash to turn the heater fan on and off. The heater hoses usually contain a shut-off valve to turn the water flow off in the summer when the heater isn't used. This valve would commonly be installed at the water pump where the heater supply hose is connected.
