Scotie, good tip. Thanks. Although I haven't ever had an issue with manilla folder material.
Marty, sorry, I think I completely missed your point. You are asking about the insulator "boxes" inside, not the wrap that is sometimes around the outside of a smoke unit. The lateral PA10519 and longitudinal PA14A209 smoke element insulating folded "boxes" that you're looking for are usually available from Doug Peck at Port Lines Hobby Supplies, but I see where he is out of the latter style. Try Tom Seccia of Triple-S train parts. He might have some.
In fact I just noticed Tom is selling a PA14A209 on eBay -- https://www.ebay.com/itm/Ameri...3:g:UdIAAOSwPCVX3sZ2