Heddman doesn't make them anymore. A few people have looked into it.
I was in touch with them & no, they will not (ever) make them again.
I have two sets. One set was on a started only engine. One NOS set.
Both will be cleaned up & ceramic coated inside & out.
Sorry, not for sale !
Best & cheapest way to replace them is have another set made, by a good builder.
I think you may still be able to buy the flat collector ?