Ratchet & Clank’s primary source of currency are shiny bolts. These come in a variety of colors and can even be changed up in their appearance by using the “Extras” menu. Bolts can be used to purchase different weapons as well as to refill your ammo and health up.
In order to get your hands on as many bolts as possible, you’ll have to destroy everything in your wake. See boxes? Bust them. See fences, lamposts, and other items strewn about a level that can be broken? Break them. Destroy every enemy and wait around their disappearing corpse for an influx of bolts. You’ll soon find yourself rolling in them. Especially in later planets where tons of enemies will be swarming you.
To help you with your quest for more bolts, the Box Breaker upgrade for your wrench will be very useful. It is acquired on the planet Gaspar by getting all the 60 brains for the Brain Scientist. It will smash all boxes and breakable items around you with a simple jump and slam. Really speeds up the process of gathering bolts around a level.