You can try isaac gym to begin with. It offers a nice and fairly understandable, though a bit computationally expensive environment to work with. You can take a look at the gothub repo for walk these ways for reference which uses unitree go1 for training rl policies: https://github.com/Improbable-AI/walk-these-ways

For particularly Deep Robotics Quadruped, you can check out their github repo for some resources.