The best way you can presently get Rocket League on Steam is via purchasing a key from a third-birthday celebration key reseller. Many of those web sites may be ethically questionable (to place it gently) so we might not be linking to  Rocket League Credits any right here, however you could easily find them with any search engine.

Another workaround you may do is installation Rocket League from Epic Games, and then upload it to Steam as a non-Steam recreation. This can be performed via opening Steam, selecting "Add a Game" from the decrease left nook, clicking "Add a Non-Steam Game", after which finding Rocket League within the file browser. You won't be able to track playtime or achievements with this technique alas, but it facilitates you preserve your video games centralised to Steam if that's Buy Rocket League Credits what you need.