Where would you source these coping stones from?

[Jamhan]

We are missing some flat coping stones from the parapet wall, as seen in this picture: https://imgur.com/a/kIEcmf9.

I've tried a few reclamation yards around us (East London), and none of them have them, nor seem like they would ever get them
How would you replace these?

 

[datarebal]

pictures not sharp enough , could be one of a few