Yes, they're gone from the map after being captured and won't respawn again. 
Thanks. :)

i think you build the lab then bring strange gold to craft them....you get strange gold from amagal country first you go to a town of that country and get uniform then you go to main base of that country and enter a hidden dungeon....you need iron you will transform it to strange gold in the dungeon....you can buy iron from elf town

good luck