I've seen some people with it, but I can't seem to find out HOW to get it... I want that Regice!
You need to scan a QR code to get it. Here it tells you who you need to speak to to get it. This video shows someone getting it.
After scanning the QR code, go to Mossdeep City and enter the house where there is an old man and a door. It will be open if you scanned a code.