A Trainer called thevaliantbacon found a trick (or guide, however you want to call it) that enables you to get free Premier Balls really easy. It’s not a hack or exploit, as it’s just using the built in Buy and Sell mechanics in Pokémarts.
How to get unlimited free Premier Balls
On the second island, after clearing the 3rd Island Trial and visiting the Battle Royal Dome, head over to the Mega-Mart directly to the Dome’s right.
When you first step into the store, the attendant hands you a coupon that returns half the money you spend at checkout. From here it should be pretty self-explanatory.
Go to the cashier that sells Pokéballs.
Buy as many individual increments of 10 Pokéballs (or 11 since it’s faster) as you can afford. For each one of these you get a free Premier Ball.
When you are broke sell back all thePokéballs – you will regain 100% of the money you spent, but keep all your Premier Balls.
Repeat until you realize that Premier Balls kinda suck anyway, or until you discover they can be sold for a whopping 10 Poké Dollars. Cha-ching! Max money, that is if you want to waste hours just wearing out the buttons on your 3DS.