Golden State Parts 2004-5 catalog has heater cores on page 44, but they are not too cheap either. They range in price from $225 to $300 and all require a refundable $50 core charge. If you are interested I could scan the page and send it to you via email. I found a heater core on ebay for my Fresh Air Heater, which is supposed to fit 51-55 heaters. The heater-core for this type heater goes for $225 + core at GS.
