Don't the Kua-Toa enemies use nets?
I dunno. I haven't played that far. All I know is that the Half-Ogre Buthnir once threw a javelin at me that I couldn't find to pick up. Maybe the Kuo Toa nets will be the same then? Unless you can mange to steal them first...
Yes, the kua-toa use nets. I don't think you can get them, at least not yet in the current iteration of the game.
*
I believe the earliest javelins you come across are in the chapel. They're on the fellow inside who can go into a rage. Is his name Haseid, I believe? Anyway, one of them is carrying two javelins.