Hi, I hope I can be of help to you on this. Please see the link to the parts list that I believe will help you in your search. http://www.heatilator.com/~/media/Files/Heatilator/Service%20Parts%20Lists/HB42A_HB42AI.ashx Secondly the only thing that the manual points to is that you need to contact a dealer, I have provided a link to distributors, all you should have to do is enter your zip code to find a contact near you. Then you should be able to use the parts list I attached and hopefully find the part you need.
Here is the distributor find page. http://www.heatilator.com/Customer-Care/Contact-Us.aspx. I hope I have been able to help you get further than you were getting.
If you are satisfied with the answer and time I have taken to help you please press the accept tab. Thanks
Sorry, The parts list does list some refractory panels I hope these are what you are needing.