You cant -_-. The only Rocket Launcher you find and get is in the Final Boss fight in Leon's Campaign. It would be stupid for it to have infinite ammo when you cant find it anywhere else but that one part.
Edit:Why the hell would you make the same damn topic again when you allready got an answer before?
on page 93 of the resident evil 6 book it said equip the rocket launcher infinite ammo skill while replaying leon's chapter 5 . the rocket launcher is then permanently added to your inventory with infinite ammo.