I like the Alicia Hair at RDNA. The default is an Afro style, but it's got a lot of morphs to make it longer or shorter, and textures for curls that are less tight. There are gray and silver textures, too, though I can't remember if they came with the hair or were sold separately.

You can also make your own white/gray texture for any hair you have, by desaturating a blonde hair texture in Photoshop.