The Wii Shop Channel was permanently closed on January 30, 2019. No games, including any Mario Party title, can be purchased or downloaded there.
Digital Wii purchases are no longer possible. If you previously bought a WiiWare/Virtual Console title, you can still re-download it from "Settings ▸ Data Management ▸ Wii Shop Channel Account Activity" on a Wii or Wii U—but new purchases are impossible.