How do you get retro cups?

the retro cups are the leaf,lightning,banana and shell cups

The cups in Mario Kart Super Circuit, Mario Kart DS, and Mario Kart Wii consisting of tracks from past Mario Kart games.

You need to play the shell cup to unlock the banana cup to unlock the leaf cup to unlock the lightning cup. this half is all retro. the other is new/nitro.

They are, in order from left to right:-Shell Cup-Banana Cup-Leaf Cup-Lighting CupI'm not sure if you were talking about the Ds or Wii version but the one I put up above is the Ds version.

a mirror cup that's on the bottom of the choice of grand prixes