First thing you'll need to do is find a car body, with doors. Most of the bodies I've come across, I find on Craigslist. Craigslist just seems to be a site where you can still negotiate a price for a car/truck compared to say eBay.
With that said, you'll need to also know your own skills. Can you weld? Can you do wiring? Etc....
Based on your skill level, you can move forward with your build, or you can start looking for something that is mostly complete. My guess on buying something built would be 3k to 15k. Most rat rods you'll find for sale on Craigslist need to be finished. People start out with the best intentions but loose interest once they hit wiring. If you can do wiring, you may get a good deal cause the owner of the ride is probably frustrated and wants to get rid of their almost completed ride.
So as you can see, you have a few choices. Good luck with your decision and build.