Simplistic question: Do I need an Eon Ticket to get it, or do I just need a Latias.
in order to get Latiasite in Omega Ruby without the Eon Ticket, you have to ask your mother in Littleroot Town and she will give it to you. This only can happen after you first clear the game.
Yes and no
yes you need the eon ticket to catch latias and get the latiasite
No you can get it from someone who owns Alpha Sapphire