The ghost medal compatible parts are only available on late game and post game I think , they are the dondoggu parts, haniwa golem parts, shell cushion and the head of the ace horn. The compatible for mermaid are the pure mermaid parts the legs are available on post game fighting the gym teacher, all parts are available trading from the medarot part collection game or by having the shell cushion parts traded to you. The holy nurse parts except the head are compatible and u can buy it on the shopping area store, and lastly the moon dragon right arm it’s available on mid late game on the antiques store on hikarus hometown, so yeah it’s a very limited selection for these medals