Hello!
I've had some problems with these thing before also.
Here I give you some links where you might find a good texture for that
http://3dsky.org/
http://www.textures.com/
Another way will be to go to the website of a company that sell those stones, sometimes they have pictures that can be use as textures.
And my last resource for this is to fix the texture in Gimp (or photoshop)
just google "create seamless texture in gimp (or photoshop)" and you'll find good tutorials on how to do it, is not difficult.
Hope that helps!