when you find one of them, there will be at least 1 or 2 other argys in the area. make sure you find one that is alone. then shoot with tranq arrows or darts. Then feed stego kibble then you have an argy! Hope this helps!
An argentavis can't fit through a ceiling with one tile missing, but a pteranadon or lymantria can. Build a trap with one corner ceiling missing, aggro the argentavis, fly in one side and out the missing ceiling tile, and land. Dismount, run around, and close the entry door. Tame safely at your leisure.