They’re Available Through Legendary Raids
Legendary Raids work pretty much like the standard raids in Pokemon GO, only they’re ruled by much stronger creatures. Currently, standard raid passes will get you right into the event when it goes live, but you can expect Legendary Passes to be required in the near future. It seems as though with the server strain the game recently suffered, the developers aren’t trying to overcomplicate anything.
The raids are timed events and require a lot of players working together to beat. You’ll get a notification for when they are scheduled (an egg will appear over a relatively nearby Gym and give you 48 hours to prepare). Upon victory, if you’re lucky enough to actually win, you’ll have an opportunity to actually catch the Legendary Pokemon. Keep in mind that they have among the lowest catch rates in all of Pokemon GO, so just because you win doesn’t mean you’re walking home with the grand prize. Bring some Master Balls and Golden Razz Berries to increase your chances.