I played Pokemon X last year, and stopped playing after a while. Beat e4, bred some nice Pokemon, but never had the courage to play online and get my ass whipped. Therefor I didn't touch the game for some months. After beating the e4 in ORAS, I decided that I want to make a living Pokedex, with the aid of Pokebank. I booted Pokemon X and before I went to the Friend Safari's to catch Pokemon I hadn't caught yet, I checked my Pokeballs. Exactly as I suspected, I had quite some duskballs and quickballs left, but also ~100 Premier balls. I don't remember buying 100 times 10 balls to obtain the Premier balls (and I don't have that many balls left). Was there some sort of event that you obtained those balls? It is plausible from the description:
A somewhat rare Poké Ball that was made as a commemorative item used to celebrate an event of some sort.
So I finished the game, but didn't participate in any events in the last ~8 months for Pokemon X, except for a Diancie and ORAS demo. So, how did I obtain 100 Premier Balls?