Research
But in regards to finding the car itself, I'd personally look at autotrader first. That's how I found my car. I don't trust people enough to buy a car on Ebay, pay thousands of dollars and then have it arrive at my door without seeing it in person.
You can find great deals out there, you just have to be patient. For example, there is a guy on another F-body forum selling a 2002 Z28 M6 with 30,000 miles for $14,000. He is in San Diego, CA - I'm not sure where you are located.

