I just got Rocket League for the Switch and I noticed I get the Mario/Luigi NSR right out of the gate. How do I unlock Samus' Gunship though?
According to this question thread, Samus' Gunship can be unlocked as a random loot at the end of an online match:
It's unlocked via a drop when you complete an online match. The drop for it is random though. Just keep playing and you'll get it.
It is confirmed by some other members, like this one:
Like others have said, random unlock like any other part. I was lucky and got it within the first 10 just keep playing.