There’s a big open area near green obelisk, redwoods and swamp. If you are lucky you’ll find one. Beware though, spinos, rexes, raptors, carnos, barys, and theris are all relatively common around here. I’d recommend a good flyer or bruiser of your own if venturing here. Flak also helps.
Not sure where this goes but if u place a spike wall in front of them and when their health is low enough they have to turn into a ball and be stuck in a constant cycle until the spike wall is removed. It's a great way if you find one and want to tame it later. Unless it gets killed by something else while in a ball it should be fine. I stil have one stuck on one of my walls and ima keep it ther until I can tame it for stones.
You can find them in the redwoods, by obsidian mountain, and north of the beaver spawn near hunter cave. I highly advise using an Argentavis so you can just pick it up and knock it out in a safer spot.