You can find these in the "Body Tuning Shop".
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2707389378
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2707389421
I assume, you're working on a customer job, but you can also use them on your own cars, that you want to flip later. They don't enhance the performance of the car, but make it more expensive (and more awesome ;) ) so you'll make a slightly larger profit.

And just in case, you'll download cars from the workshop later on: you find these parts in the "Community Cars Body Shop" (fresh import goods).
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2707389484
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2707389529

Yep, working on a customer's car, and it won't leave until I buy these parts. Guess I need to get the tuning shop DLC then?

Yes, you need the tuning-dlc. I keep forgetting, that it is a dlc... There's no need to not have it, since it's free.

If it's not installed, which happens to people kinda often, then go int steam library with game selected and click the manage button then manage dlc and check the box next to it. It would be in the list cuz it's free.