The following resources are totally free, including for commercial use, with the general exception that you must not redistribute the texture on its own, certainly not on your own texture distributing website.
http://www.mayang.com/textures/ (20 downloads a day)
http://www.grsites.com/archive/textures/ (only brick and rock is relevant to us)
http://www.imageafter.com/http://textures.forrest.cz/http://www.tutorialsforblender3d.com/Te ... index.html - Site contains lots of stuff on Blender, not just textures. No explicit licence declaration.
Here is a tool that promises to help you create your own Wang tiles, but I never tried it out:
http://robburke.net/mle/wang/Wang tiles are what the RW terrain textures use, a scheme for more interesting and still efficient textures.