Where can I buy Heat-n-glo control knob?

Where can I purchase a replacement control knob for my Heat-n-glo gas fireplace, model SL-75-TR-D. I checked www.heatnglo.com but I don't see anyplace where you can order parts. This knob is plastic and has cracked in half. I called a local fireplace dealer and they said I'd have to buy the entire valve, which is approx. $300 installed. Anyone know of a place to buy parts?