Or learn how to make your own.
Take any photo, cut out a good even squarish block, and get it to the resolution you need. Then if you select half the image, like split it in half vertically, you can mirror both halves to split the seamless line. It'll make a horrible seam in the middle that you'd get rid of using the clone brush. Once you'd tidied up the middle, you do the same but split it horizontally, do this a few times until you can't find any more seams, and you'll have a seamless texture. It's really easy with gravelly textures, rock is quite tricky though.
Van-B

Put those fiery biscuits away!