If you do daily challenges, you get shipments quite regularly. Also the later you get in the game, the faster the cars will get. This means that you will be able to get bigger scores and level up faster, getting even more shipments. I started noticing this after all my cars were car level 250+. Played 30 minutes, collected 4 shipments. Currently level 24, got my first red underglow and also have 3 air suspensions.
It would be good to be able to buy them, but then again they are currently built into the loot crate system (shipments) and loot crates in general are horrible (look at the Battlefront 2 scandal for example)...