Rocket League credits can be bought from third-party platforms or directly in the game. However, there are also some ways to obtain them for free if you find the right methods. Now, you have to bear in mind that you're not always guaranteed to get credits for free, so it's important to spend some time doing research.
You can get free credits in Rocket League by doing Rocket League item trading. You can trade many things, including free drops. Obviously, you need to send invites to players you want to trade with. Also, before you do the trade, you must agree to the trade's details. Otherwise, you will probably experience some unpleasant situations.