The game has crowns and battle crowns marked so they don't appear in shops or battle loot. Each king/queen has their own crowns. If you want to be able to purchase them, you will have to change the xml. There is a file that contains all head armors. Find the crown(s) you want and change the attribute is_merchandise="false" to is_merchandise="true". Then you could obtain the crown, by chance, when defeating the king that wears it or eventually it will show up in one of the towns for you to purchase. You can speed up the chance of it showing up in towns by changing one of the shops in towns to a Smithy. While you are in the xml, look for the culture attribute of the crown you like. Find a town, in the culture that the crown will originate from, with the highest prosperity, and buy a shop there and change it to a Smithy. Keep it or sell it after you change it. When I get some free time I'll make a mod for this. Or someone who reads this can make one...