Lots of ways to get them. One way is build a trap near argies and use yourself as bait. Stand near trap entrance. Shoot one with tranq dart. It will aggro on you. Lead it into trap as you run out the other side. Walk around to shut door. Done. Make a saddle before you leave ...
Now, get the Argentavis to aggro on you by flying past its face with your mount (Other nearby Argentavises may also aggro on you, so try to get them to de-aggro or kill then while you lure the only one you want!). Lure it up Far's Peak into the crater in ...