Think this another location from Onegai Teacher. Near the Lawson there's a Yupuru complex, Yu=bath and Puru=pool. As the other location hunt site says: "Whenever you see the cast bathing together in the series (except in Maiku house), they are bathing here."

Of course during my stay at Kizaki I went to the onsen here ^_^ Since it's rather strange to take your camera inside to take pictures, I made some sketches of the place. It's been a long while since I touched a pencil, so please bare with the crappy sketches.
There was an inside as well as an outside onsen. Think the water was natural, source is about 10km away (water goes through pipes). Not completely sure about the water, but since the hostel I stayed pumps water from the source far away I assume they do that here as well.

Useful to know, the place closes at 9:00 PM. Think the opening time is 8:00 AM, but the closing time is more important. Don't be so unlucky as me and walk into the wrong building, ask for the onsen, get directions, and hear that they are closed.