Is Rocket League available on physical media, or is it only the digital copy?
Psyonix's Vice President Jeremy Dunham recently announced that Rocket League will be coming to stores soon.
Unfortunately, no price point or released date has been given.
Edit: This information was true at the time, but Rocket League has since been announced to arrive in physical form (see Powerlord's answer).
You cannot (as of yet).
According to the devs (source), Rocket League is download only.