Well, the Gnomon ZBrush Alpha Library dvd has a decent set of alpha skins included (~300 assorted scaly, leathery and bumpy skins). A great way to get the same effect and not spend money is to find reference images of skin or scales or whatever, take them into your favorite image editor and make it a grayscale image.
Taking it a step further, take a fuzzy brush and erase the hard edges around the image - it makes for a more 'tileable' map.
Happy modeling