Despite them not being officially supported, at least not to a significant degree, Rocket League still has a rather large collection of user-made obstacle courses and training arenas. Once the upcoming December update rolls around, however, you can expect to see that number go even higher as Psyonix has announced that Steam Workshop support is coming to Rocket League!
In other words, you will soon be able to easily browse through all of the currently available mods, pick and choose your favorites, and then add them to your game with the single press of a button. Once downloaded, you can access them in the 'Extras' section of the main menu and play to your heart's content.
But even if you have no experience with modding, worry not as you will still be able to create your very own training scenarios with the upcoming "Customized Training Mode". Long story short, you'll be able to create a variety of challenges, combine them into a training session, and then easily edit all of that until you are perfectly satisfied with the results. And best of all, you will also be able to share and download these training sessions via the Steam Workshop!
As for when exactly this update will arrive, I'm afraid Psyonix hasn't announced that just yet, but you can expect to see at some point in December. Until then I would recommend keeping your eyes peeled on the Rocket League website as Psyonix has promised even more revelations and details over the coming weeks.