Cloacks has shared on his Deviant Art profile a pack of 8 marble textures with different grain that we can download for free on your profile in its free version.
The textures have measures of 800 x 600 pix and asks us that if we use them we make reference to it as stated in its terms of use.
This pack of marble textures also has its paid version that corresponds to the same images but with a larger size (4000x3000px) and that has a price of $ 3 that we can pay by Paypal.
Source | 8 marble textures