I was the original poster on this. I was never able to find that part anywhere.
What I did was just to cut a piece of rubber that was about the same size as this part. The same size of the section of it that is the actual seal, not the part that goes through the slot to lock it into place. Just a rectangular piece of rubber big enough to block the airflow the same as the original piece did. Then using double sided tape, I stuck it over the slot. I wish I had a picture to show you but I don't. it doesn't have to look like the original part, it just has to do the same job.
My heater works fine with no problems and no one is ever going to know you don't have an original part in there.
Dave