You can buy tickets online at CKS, no reason to involve a third-party.
https://www.cksp.com.hk/en/ticket
But you don't really get a ticket, just an order confirmation which you need to exchange for your ticket. Presumably with your passport, so I doubt an agent could get you the actual ticket in advance, either.
Whether an airport check-in clerk would accept the order confirmation is an unknown.
This was sort of my understanding, though I lack personal experience. Honestly, I have to assume that the order confirmation serves as "proof" in the same manner as a ticket number does for flights, but since these are uncharted waters,
Nabi might derive additional comfort from getting an actual ticket in hand before leaving Korea.