Hi,
You can try our list of onsen at
www.secret-japan.com/onsen
Use the Search engine and choose your region : Tohoku and push the mixed bath button
(I got 5 answers, 2 in Akita, 2 in Aomori, 1 in Fukushima).
I have no place for wild onsen in nature but I know there are alot, if somebody knows ?