Custom flag stitching is (was?) an option offered by Erika on her line of Erika's Originals MN (Marine Nationale) straps
https://erikasoriginals.com
I suggest just email her with whatever you are interested in.
(The email contact is at the bottom of her website)
I cannot find a reference to the stitching on her website right now,
presumably it is one of the paid options, but I cannot recall how much
There are some other examples of flags in her Instagram feed
https://www.instagram.com/erikasoriginals/
cheers