the best thing you can do is ... catch pokemon, rinse and repeat several times against gym leaders and such you've already beaten (you still get exp and pokedollars) and raise your pokemon until you have a satisfactionary team of lv 100's. That's how I did it, though I'm currently doing some legendhunting before I rly start cracking at the sidequests.
You can also always take a gander at the poké auctions and ask around for certain trades that you might want to make or have on offer.