Where do I get car parts?

I believe what you are asking is how to get the parts to build the toy car. If that is your question, you need toy car tires, the only way I have been able to get them is from chests. Chests are won as part of completing missions in the game, occasionally you can finish a daily task and get a tire from one of the boxes you get in exchange for a merged item. I hope this helps

You can get car parts from the car symbol in the game when it flashes with the lightening bolt saying it is ready