Railroad tracks are also pretty good, depending on the location, the fill around the ties can have really good sling stones. They are usually about the right size, but in some areas they are much too jagged. The target should be able to walk over all the rocks if they aren't too big.

I've been really enjoying having a backstop. Using a baseball field with rusty balls has been ideal. Even using the underside of highway/street overpass would be a good backstop.