In Shiobara there are a number of onsens that are not part of a hotel or onsen house. They are just sitting there in public space. Sometimes you will be walking along a track and you will find a hot spring sitting there.
So far all that we have seen have a little money box where you can deposit a small amount of money for using them. Our favourite is at the Northern end of Shiobara across the river. Its in a beautiful location by the river. Its mixed and not very well protected from viewers.