Find stock cars for sale by browsing auction websites such as eBay.com or classified ad websites for vehicles for sale such as Cars-On-Line.com. You may also try visiting a local race track and asking whether anyone might be interested in selling a stock car. Do a fair amount of research on the stock cars available so you can get a good quality car at a fair price.
Identify where you want to race before buying a stock car. Visit a local race track, observe the cars that perform well, and see what type of cars they are. If you can't find a car that fits your preferences at a particular track, ask around about any other track where you can find the type of car you want, then go to that track and start doing your research there.
When looking at a particular stock car for sale, be sure to ask who built the chassis, who has driven it and how it performed in its last few races. Racers come and go, so there's always the opportunity to get a race-ready stock car at a reasonable price if you are willing to invest some time and effort towards learning more about the vehicle you are interested to buy.
One way to save money if you have basic mechanical ability is to build your own stock car. Aside from savings, this helps you gain more intimate knowledge about the stock car you intend to drive.