The popular RL items that fit under the cars, decals, wheels, and goal explosion categories can be obtained via many different methods. 

Playing Matches– Rewards are given to players for playing matches. These rewards come in blueprints (especially for rarer items). Players then must unlock the blueprint with credits to obtain that specific item. 

Player to Player Trading– Trading in Rocket League is a common way to get new items. You may have an item worth a certain amount of credits which you can sell to then afford your desired item.

Item Selling Websites– Websites such as Aoeah sell Rocket League items for cheap prices with fast delivery times. You will generally receive your items within 15 minutes depending on the time of purchase, what item, and which company you use.