But the New York Times insists that they obtained the weapons on their own from Syrian warehouses

""Syrian rebels have acquired as many as 40 shoulder-fired missile systems in recent weeks to counter assaults by Syrian military aircraft, introducing a possibly decisive new weapon into the conflict, Western and Middle Eastern intelligence officials say." "At least some of the missiles were supplied by Qatar, which has provided most of the weapons smuggled to Syria's rebels across the Turkish border, according to two Middle Eastern intelligence officials briefed on the matter."" (thanks Amir)