Sources for Free Soil
A favorite site for bargains, surf the "free" section or post a wanted add for clean fill. People are sometimes surprised at the amount of dirt they need to get rid of when doing projects, and it could be your free way to a container garden. You might also ask people if they have dead plants in pots they want to get rid of. Since these are usually a giant chunk of dirt and root when they come out of the containers, you can easily cart away the "icky" part that can become great soil with care and some extra compost.
These will be more of a challenge, since it's likely they'll want to keep dirt for themselves for future projects. But it's worth a phone call to the companies listed in your yellow pages to see if they have extra dirt from a recently completed project that they'd be willing to part with.