There is no explicit option for classic cars on Craigslist, but the most efficient ways to find them are to use model year restrictions or to use the word "classic" as a query when searching. These features are both available on the Cars and Trucks page.
For any localized Craigslist site, the link to this page is usually specified under the For Sale category. The model year restrictions are two of several possible search refinements available, including make and model, price and odometer ranges, and other options, such as opting to view only results with images. Users can also choose between vehicles from owners, dealers or both.