The tickets drop randomly from bosses, but it's only a chance.
Fortunately they also drop from any mob, so try going to a delve and killing some mobs. I've usually got mine after killing 10 mobs, but it's pure chance
If there were more than 12 people fighting the boss, you might not get any loot. That's a general loot cap that exists everywhere except for dragon fights, afaik.
Fortunately, you should still be able to get your tickets from bosses or mobs that you kill, just remember to loot everything.