A long time ago I used to paint Warhammer fantasy figures. Several of the bases I set as cobbles (being a medieval fantasy world, it looked just right).
To do this I spead a 1-2mm layer of Miliput or other malleable putty on the base, then using a small tube - I can't remember what, I think it was the end of a biro refill or similar - pressed it into the putty and it makes small domes, which can then be painted in greys and browns to look like cobbles.
If you wanted square cobbles, I'm sure it would be easy to make a stamp out of a bit of rubber or wood. You can also do patterns like fans, or gutters etc with a bit of planning and experimentation.
Good luck!