How do you get a sun ribbon on Pokemon Mystery Dungeon to?

1. Get the Sunny Day Ribbon (from the post game Special Episode "Heat Up! Solar Flare Showdown!".).

2. Get the Rainbow Ribbon (Get your team's friendship to max, then go talk to Wigglytuff, this ribbon can only be unlocked in post-game) .

3. Go to Mt. Faraway, take the path into the volcano. Defeat Groudon with a Fire, Water, or Grass type move to recruit him then head back to Wigglytuff and he will award you with Sun Ribbon.