I was looking a bit about for manga's tones more in a way to research a bit morse about it.
and found this 2 sites how had free digital textures to use. I don't know if it is really suable but is sure nice to look at it and let it inspire you to develop your one tones designs.
and the Psychobob site haves some links in the bottom with other links to manga stores and tips and things like that
Link to Shounenaigo, psychobob