Here are some background sources, note that some are offered as linkware, meaning you have to display a credit return logo to their site.

Marble Textures at Absolute Background Archive ~ my personal favorite, great list of backgrounds, especially marble. Some very well made.

M A D H A T T E R backgrounds ~ A very lengthy list of backgrounds, has a marble bunch too. Nice but not very natural looking.

Background images at TMBFree ~ Nefermaat clued us into this one, some really fantastic marbles and other backgrounds.

Backgrounds from Letty ~ Foreign site, some really unsusual but original backgrounds in there.

Cheri's Glorious Heaven Graphics ~ no comment on the name there, but she has a marble page, and some interesting backgrounds.

Textures by D'Designs ~ Just a nice (well made) textures display.

Creations of Paradise ~ Some nice muted backgrounds in marbles and textures.

Enjoy!