Hello!
For the issuer “Livonia and Riga” (livonia_and_riga_monetary_union),
The flag should be a .gif of what is currently shown under the Livonian Order:
https://en.numista.com/catalogue/livonian_confederation-1.html
And the flag that is currently shown under the Archbishopric of Riga:
https://en.numista.com/catalogue/archbishopric_of_riga-1.html
This was the monetary union of these two places, so I think a combination of the flags would be best. :)