I just purchased a new home and it has a fireplace with a cracked back panel. Does anyone know if there is a product that I can use to seal the crack?

So you are talking about a pre fabricated fireplace like a heat recirculator, right? The crack on the panel my not be as serious as you think since the flames to not actually get thriugh the crack to the heat exchanger. You can buy a replacement panel from the manufacturer or try to patch with refractory cement. This may be a 6year patch job, cheap easy but yearly.

Michelle you stated that you just purchased the home, Right? I had the same problem with my fireplace. The builder of my home gave me the installation manual on the fireplace. The fireplace has a 1 year warranty. I called the manufactor and they sent a new panel. Maybe something to think about.