There is a member selling a parcel shelf on here but it looks like that piece is missing on his too. The only chance you would have is if you can find a car in a breakers yard. It's unlikely to be a piece sold on its own
Have you seen the one advertised in the for sale section on here, it's under the heading "197/200 parts for sale" the lad selling it is wanting £150 but like the one you've seen it looks to have the end trim section missing. I don't think it will affect the operation but given it is visible it needs to look right. I would hold out for a complete one.