I’ve found two seals. One is in that tower in the same town Kang Wang starts the game living in.
The other was in the command tent of the Yang army camp and you have to steal it. The camp needs to be discovered by having high fame or doing the Ye Family questline.
There might be other ways but those are the ones I’ve discovered. The Ye Family route makes them hostile. The Fame route lets you talk your way in.