I have no idea what the liner in your insert looks like. If you're motivated by the price of the replacement OEM liner, I'm guessing you could DIY one. Option 1 is to buy firebricks at a masonry supply house and rent a tile or cutoff saw to cut them to fit the sizes you need. Option 2 is to buy a bag of castable refractory cement and build a form to match the shape of what you need to replace. Then you cast the replacement and put it in your fireplace insert after it has cured.

Either is likely cheaper with your sweat equity/time investment than buying the OEM replacement.