There are two places to get free Poke Balls daily in Sinnoh - use them!
Celestic Town - PokeCenter
There is a man here who will give you a Great Ball once per day if you show him a happy Pokemon. Not great, but not too bad either.
Solaceon Town - House directly east of PokeCenter
The news reporter here will ask you to bring him a specific Pokemon once per day and give you three Poke Balls as a reward when you bring it. He'll only ask you to show him Pokemon that are already in your Pokedex, so it's never too hard. We like to use him as a way to push ourselves to catch 'em all. However, if you just want the Poke Balls without a hassle, you can save your game before talking to him, and then if he asks for a Pokemon you don't have, you can just reset and try again, and he'll ask for a different Pokemon. You can repeat this until he asks for one you have.
Similarly, you can use this technique to get more valuable Poke Balls out of him. When you have the Pokemon ready to show him, save your game. Then, if he gives you some lame Poke Balls or Heal Balls, you can reset and try again until he gives you something more valuable, like Dusk Balls or Ultra Balls. The types of Pokeballs you can get from him are: Dive Balls, Dusk Balls, Great Balls, Heal Balls, Luxury Balls, Nest Balls, Net Balls, Poke Balls, Quick Balls, Repeat Balls, Timer Balls, and Ultra Balls.
Above: You'll never have to pay for Poke Balls again if you hit up these two places daily