Rivet Town: Rodney's dad gives scrap
On the first level, after you have grown up, go to where Rodney's dad is located. Once you are in the diner in the room where he is found, he asks you to hold L1 and select the Scrap Launcher. Select it, but do not shoot targets yet. Instead, shoot everywhere else, but not at the targets. Use up all your scrap, and Rodney's dad will give you 10 extra scrap.