I don't think you can buy wood ashes. You have to make your own. Quiet frankly, wood ashes are very dirty. We have a wood stove here and I doubt I would ever let them dust bath in them. I used to dump wood ashes in the garden area that is now the chicken area, and I am dealing with them eating the charcoal pieces now. Charcoal absorbs nutrients in the system, so I don't want them eating the stuff. So I am constantly picking up charcoal pieces. However if would like to try them out, maybe you could ask a neighbor that may have a wood stove and get a bucket full to try out. Anyone with a woodstove, has more wood ashes than they know what to do with
we heat with a woodstove... so I have had plenty of ashes to dump over the years.. when they get wet they get "slimy" for lack of a better term.. my chickens won't dust bathe in them (which I am glad of since the ashes are so messy)
A lot of animals will eat charcoal, it helps get rid of toxins in their systems. My chickens love to dust bath in the wood ash I pull out of the BBQ. Yes, for a day or two you have a bunch of gray chickens running around, but they seem to enjoy it and I am sure it helps to keep the bugs off them. They do not get in the ash when it is raining so no problem there.