Just make the boss seller sell units like a mercenary camp. Then use a trigger that runs when a boss is sold to move the boss to the correct location.
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here is the map with the trigger, it is currently disaled by accident however if you place it into your map the exact same it will work just fine. If you want to make it so it is a different area spawn for different bosses, you will basically be repeating that trigger, or if the bosses can only be hired once you can have a action to close that spawning point and open the next spawning point for the next boss. All in all if you follow that you are good.
i did exactly what your OP was asking for, now your asking for something completely different which is hard to tell because of your poor spelling capabilities and grammar.