There are many resources for finding sento and onsen throughout Japan. This is by no means a comprehensive list, but these are the sites that I use or have looked over.
Onsen Meijin [ Website ]
Decoding Kyoto: The Sento [ Website ]
Onsen Blog by Sonoe [ Blog ]
The Way of the Hot Springs [ Blogspot ]
Beppu Navi [ Website ]
Gensenkakenagaski.net [ Website ]
Michiken Land [ Website ]
Nikkan Sports - SentoNavi [ Website ]
Supersento [ Website ]
Nyuyoku Report [ Website ]
YuNavi Kansai [ Website ]
Hyogo Prefecture [ Website ]
Osaka Prefecture [ Website ]
Kyoto Prefecture [ Website ]
Shiga Prefecture [ Website ]
Okayama Prefecture [ Website ]
Nagano Prefecture [ Website ]
National Public Bathhouse Association [ Website ]
まっぷる 関西 日帰り温泉 [Mappuru Kansai Higaerionsen] published yearly
るるぶ 日帰り温泉 関西 中国 四国 北陸 [Rurubu Higaerionsen Kansai Chukoku Shikoku Hokuriku] published yearly