You can easily find then in high mountain-areas bring a pteradon as your back up the shoot it with tranquil arrows/darts and bring steggokibble and a few narcotics just in case. Then protect your tame so it does not fall as prey to other carnivores. Always start with a low level then once your level'd up go for the higher ones. Don't forget to bring the saddle!. And lastly please make sure your tough enough to face the surrounding area
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.