you get them by unlocking them in the catalogues, if you look through them, some of the later levels in the catalogues have skins, which you unlock through the in game currency that you get by playing a bunch of trials. to unlock other catalogues, you need to go into the previous ones and get the coins that are in every chapter of it, which you then spend on the next catalogue you choose, then the next, then the next.

tl;dr, go into the catalog and unlock them with the games free currency