There’s a huge modding scene for Mario Kart Wii, and lots of custom tracks. There’s also several track compilations. One of the most well-known is Wiimms Mario Kart Fun which currently has 449 custom tracks. It runs on a real Wii as well as in emulators. My wife and I used to play it a lot.
The tracks are made by various community members, but the compilation itself is made by Wiimm, the same guy that made the Wiimmfi service that lets you race people online even after Nintendo shut down the official servers.