It will probably be the first quest, with some 'revelation' or similar that the guy who has your light sabre is a Sith. Then you'll kill him to get your lightsabre back.
Or something. I hope it isn't as utterly predictable as that...
That would make the most sense to me.
Or maybe you don't meet dark jedi early enough to swipe one?
Aren't they supposed to "personal" ?
Or can any jedi pick up any lightsaber and use it, just like a screwdriver ?
Seemed to be able to in that Clone movie.