You can't get new courses on Mario Kart Wii directly. The game comes with a set number of courses that can't be added to through official means.
However, there are a few ways to experience more tracks:
Remember, the original Mario Kart Wii comes with a fixed set of courses. While you can't add new ones directly, there are alternative ways to enjoy more tracks, though some options require modifying your console, which comes with inherent risks.