To make the mold someone would have to make (by hand - or use a 3d printed model) to form the master to cast the body from. That's a lot of skill and time.
For 3D someone would have to make the 3d file (mesh). Not an easy task to do from scratch. Many of the things you see 3D printed on ebay are by people who bought OTHER people's or companies files and then printed themselves. They don't make the files - they just buy one that already exists. Some 3d files can be hundreds of dollars.
Making the 3d file takes a lot of skill and a lot of time. Meaning either way would be expensive (if a 3d file did not already exist).
You may want to see if you can find the vehicle you are looking for first (an STL file) then you just have to find someone to print it for you.
For instance - https://cults3d.com/en/tags/1:24 or https://www.cgtrader.com/3d-print-models/1-24 or many other sites online.
Some files are basic (cheap) and need a lot of tweeking to work correctly and have little details. Some are expensive but ready to print with very high details.
Hope this helps to start with.