Other than by hand, with rocks found from a quarry nearby 
Are there kits already in existance I can buy, or is there a quick way to make them?
You could try using milliput (or any other two-part water-based epoxy putty), rolling it with a rolling pin or a bottle with a separating medium such as wax paper, and then scribing in the joints.
What scale are you thinking of?
split peas or lentils work great. Glue them to your surface then fill them with spackle leaving enough exposed to represent your cobblestones.
Cork, like you would get from a craft store or DIY pale, cut and glued.
lay down a layer of plaster and scribe your pattern.
Doll house supply places make a great cobblestone pattern that is a thick latex. It can be cut to shape and painted.
Several companies make sections of cobblestones, with different surfaces and patterns, from resin and plaster.
I’ve been wondering about this myself. Anyone know of a place online that may have moulds for sale…?
I’m lucky enough that I have my own mould! [:D] Another place to check out is Kancali, all latex street sections, roads, etc. Tried it, great stuff to work with, just cut and glue down. I used tile grout to fill in the spaces between the cobbles, and it’s nice and light too!
zokissima,
http://www.hirstarts.com/molds/molds.html
Lots of potential fun here. I also remember a vendor at one of the shows that had whole street sections of cobblestones in 1/35, 1/48 and 1/72. Let me check around with some of the guys in my club and see if they remember who it was and I’ll let you know.
Verlinden sells 1/35 scale bricks by the box. I bought several some time ago intending to use them as blown out building debris, but I never got to that dio (a little too ambitious for my newbie abilities). Now I’ve decided to use the same bricks for a short cobblestone walkway on a much smaller vignette. I toyed with the idea and placed them together to see how they’d look as cobblestone and they work really well!
Eric