Small list of retailers I have seen that carry size US12 11bybbs salomon collab shoes:
https://nomadshop.net/collections/11-by-boris-bidjan-saberi/products/ https://hbx.com/brands/11-by-boris-bidjan-saberi/categories/shoes https://immense.tw/en/designer/11-by-boris-bidjan-saberi/95/
https://www.brownsfashion.com/uk/shopping/man/boris-bidjan-saberi
Farfetch also seems to have UK12.5 (US13?? dunno if that is correct, currently contacting their support to verify)