Do you have a picture of the style of cobblestone you're looking for?
This company makes metals and laminates for interiors, but I can't tell what scale the cobblestone is. I'm guessing the pop-up windows are close to actual size. Check #217, #218 and #221.
http://www.cpsgarten...-200-Series.php
Let me know what you come up with - believe it or not, I've had the same idea for years of using cobblestone surfaces for some of my classic builds, including the Bentley.
One of my thoughts was to print out to scale the kind of pattern I wanted, tack it to a plastic sheet or a very thin layer of quick-set, fireable modeling clay, then start using a scribing tool. I used the clay process to help my daughter put adobe bricks on a scale hut for a school project and it looked great.