There's many "parts" to 3D printing - I'll try to start a couple threads that will tackle some specific topics.
I have usually made my own models with a CAD package I'm familiar with. I'll try to cover software (and file types!) in another thread (unless somebody else jumps in)! But on the odd occasion I've grabbed models from a number of free and pay sights on the web.
Some of these - you will need a log in. But just try to find the search bar and type in what you want "Ford", "House" "nascar" "f1" etc
The first 2 thingiverse and grabcad are the worlds largest for free models. You will probably find a couple free models that people may charge $100 through Shapeways for..... Some of the other sites you need to buy (though some are free)
https://www.thingiverse.com/
https://www.grabcad.com/
https://www.3dcadbrowser.com/
https://www.3dcontentcentral.com/
https://www.turbosquid.com/
https://hum3d.com
https://make.dmm.com/search/c_miniature/ ~ Japanese site, google can translate it automatically
I'll point out some interesting ones to me....
https://www.thingiverse.com/tag:Slotcar
https://grabcad.com/library/mach-1-mustang-1
https://make.dmm.com/item/466312/
https://hum3d.com/3d-models/ligier-jsp217-2017/
If I missed one - please feel free to add - there are a few out there, but those are the ones I have bookmarked!
Models are one piece of the equation of course - and within models there are multiple file types depending on your tools (software, hardware and brain). You have to know what your getting and its limitations. There are some models that can be printed easily. Some require work...