At GameStop, sadly
After the month of Jirachi, Darkrai is poised to take over as the mythological Pokemon distribution event. He’ll be available from May 1 to 24, and sadly, it’s another GameStop joint, just like Mew was a few months back, so just go in and grab a code and you’re good. If you do spring for a mystery gift instead you’ll find Zygarde.
It sounds simple enough, but I highly recommend going in as soon as it opens today if you can — many stores were sold out of Mew card stocks for whatever reason (employees or friends and family taking a bunch). If that happens, the friendly Dtoid community usually posts some spares in the comments for related stories. Good luck!