So ive found a lot of rex's either by south zone or where snow meets sand it west. A great way to tame it is get a friend with a bird, have him/her fly you over (bring spyglass to scout levels) then go to town with narco arrows, etc
If you’re in the snow biome, have a decent and fast argy, and a safe place to k.o., this is a pretty simple tame. Get on a ledge, knock it out, and then just kill anything that gives prime. Argies, purlovias and mammoths are just a few of the animals that drop prime. Keep loading it with prime and eventually you’ll tame the dinosaur.