Try local creek beds. Merri Creek has a good few in the shallows all along its length. The creek bed below Rushall Station has a gully leading up towards the station and could be a good source. Also the creek below Ceres. Also railway lines - the Mernda line north of Croxton has lots of rocks lying around the siding. They’re blue metal - like the sleepers are bedded into. Dunno if such would be pet friendly.
Please don’t go out to the rock fences in the north - these are valuable reptile habitat and the local herpetologists will burn you in effigy if you nick any rocks from those fences.