On a stream with KindaFunnyGames (Colin and Greg) Jeremy Dunham from Psyonix, announced that Rocket League will be receiving a physical retail copy. Jeremy also stated that the game has sold over 2.7 million on Steam and 1.7 million on PS4, this is without PS+ with that it goes up to a combined 10 million copies of the game out there.
They had also talked about crossplay with PC and Xbox users, but that would require them to put it on the Windows Store and that would fragment their users. An easy way around this would be maybe that Steam users get a Windows Store code. Anyway I’m just rambling.
You can get Rocket League on Steam, PSN, and just recently Xbox One.