The AuroraTicket ticket for it is given via nintendo event. Unfortunately, that event has passed and you cannot get it. Note that Deoxys, being a event exclusive pokemon, is not required to complete the pokedex. But if you want it just so you can have Deoxys itself, then you have a few options:
-You can go onto a trade forum like the Serebii Trade forums and find a deal on a Deoxys.
-As a last resort, you can use a cheating device in order to obtain the ticket or just Deoxys himself for that matter.