Have you fought the trainers at Nugget Bridge yet? If you haven't, you can go fight them. After beating the 5 trainers, don't fight the Rocket Grunt just yet. First, go to the PC and Withdraw the weakest Pokémon you have and Deposit everyone else. Go to the Rocket Grunt and he'll give you a Nugget, then battle you. Lose the fight, and then go to him again. He'll give you ANOTHER Nugget and then fight you again. Repeat until you feel like you have enough Nuggets. Sell them for a fortune. When you're done, just Withdraw your team and then defeat him. Note that this is only for FR/LG and does not work in the Gen 1 games. Hope this helps.