They do still have some facilities (Storm of Iron, Dead Sky, Black Sundeals a bit with this) and capacity to 'recruit', the legions had massive amount of resources and while most of it was left behind during the Scouring a few such ships and equipment is bound to be left within each legion so that they can raise new companies. Other means are by simply recruiting existing Marines, renegade Chapters have means to produce their own Marines and might take on some work from a legion to supply their ranks. Then, as Wyddr mentions, there is Fabius Bile who is the prime source amongst the legions for new members given his skill and expertise and he has been doing this since the Horus Heresy.