1. Retail stores: You may still be able to find physical copies of Sonic and the Black Knight at some retail stores that sell video games. However, since the game was released in 2009, it may be harder to find.
2. Online retailers: Many online retailers sell Sonic and the Black Knight. Some popular options include Amazon, eBay, and GameStop.
3. Digital download: You can also purchase a digital version of Sonic and the Black Knight from the Wii Shop Channel. However, the Wii Shop Channel is no longer active, so you would need to have previously made a purchase or have access to a Wii console that still has the Wii Shop Channel installed.
4. Used game stores: You may also be able to find a used copy of Sonic and the Black Knight at a local used game store.