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 ...
Use gfi rexsaddle 1 0 0 and use this to get the same rex saddle for all rexes. Just so you learn what you will need to do a boss fight. You might only need 100 rexes who knows. Command for leveling the rex is addexperience 10000 0 1 used while riding the rex. Or take your ...