Hello.
Ragnarok has a ton of high level spawns for dinos. The center also has a large Rex spawn area at the Lava Island by Red Obelisk. Spawns in general are super long on single player to refresh because it shuts down when you log off as online the server is always going. I recommend either respawn all dinos through a command key or a PvE official server. People there are super friendly and love to help new players get a hang of it all. Its tough to find a spot to build as everyone puts pillars down to hold areas around they're base but when you find a spot it will all fell way more alive.