For my needs, I bought a pack of Plastruct's "Polished stone" sheets for 16€. Pricy, but it comes with two sheets which should suffice for the basing of dozens of minis. The sheet I got was G-scale (1/24) so the individual stones are a bit large and thus look more like flagstones than cobbles, but a little exaggeration makes it a) easier to paint and b) more visually distinctive.