If easily available, custom levels for Rocket League sound like a good idea, then you’ll probably want to look out for the game’s December update. That’ll be adding Steam Workshop support to the already absurdly well supported title, providing a central hub for all Rocket League mods.
There’s already a bit of a Rocket League modding scene, which has produced things like this Obstacle Course level. Workshop support should help nurture that little bud of creativity into a great big blossoming plant of user-created works.
Sharing and downloading new levels (or whatever else) through the Steam Workshop will work in just the same way it does for all other titles. For most users, that means pressing ‘subscribe’ on stuff you’d like to try, and having it download automatically.
There will be more details as the update comes closer to release, which is, as mentioned, currently scheduled for December.