DE is a powdered mineral and not at all toxic. Except when you breathe it in. It is tiny pieces of silica I think it is totally useless have never had it help with anything. Just another hippie froo froo fix that does not a thing. In my opinion. To kill mites you have to smother them, or poison...
Easiest way to get wood ash is to find some wood, hollow a pit out on the ground and burn the wood...then let it all burn out and cool off and then just shovel out the ashes and use them for whatever you want...Lol. Not hard to come by.
So burn you some wood and use the ashes for a dust bath...
Poultry or sevin dust really are the best answers. Although I do keep a pan full of wood ashes in the coop all the time...whenever we empty the ashes it goes straight to the coop...anyway we have a rooster that doesn't dustbathe in my spot I made...I assume because he is too big and just can't...