how do you get mega hawlucha in legends z-a

You need the Hawluchite, there's no other way. Which mega stones you can get and when is tied to story progress, so if it's not showing up for you yet you probably just haven't unlocked it. Check whether the stone shop has it listed before you go hunting, a lot of people assume it's a drop when it's just sitting there for sale once you're far enough in. After that it's the normal thing, give it the stone, mega evolve in battle.