I doubt they will let you bring your own lightsaber IN the park for safety reasons (and .. let's be honest .. they want you to buy one there).
My son got one at DHS at Tatooine Traders. The Build Your Own Lightsaber thing is pretty cool and a better quality lighstaber than most I've seen in stores anyway.
They let you unfurl it there for a photo op or two right there, but tape it up (so it doesn't extend out) for guest safety.
My son was disappointed he couldn't fully play with it at the park. But, since it still makes sound and lights up, he was still happy.