Streetpass Puzzle swap: Where do pink pieces come from?
The first time a panel with pink pieces is unlocked, you will receive a free random piece, which can be a pink piece or a regular piece.
Apart from that, meeting other players with those pieces, or being a special employee at Nintendo, there is no other way to unlock them that comes to mind.
When you get a streetpass from a Nintendo worker, they should have all the pieces, so you should be able to get a pink pieces.