Here is the web site of Yamato no Yu (大和の湯) in Narita (成田) : www.yamatonoyu.com/eng/index.asp
You need to take the subway to Shimouza Manzaki station (JR Narita, 5 minutes from Narita station) followed by a (short ?) walk. There is a map (also in English) on their web site.
This place is kind of a super-sento, rather that a traditional onsen. The outside bath seems small, as seen on this page : www.joy.hi-ho.ne.jp/ma0011/T-Chiba05.htm
Note also that while this is a real onsen (because of the water composition), the water is gushing at 18 degrees Celsius, and has to be warmed. But well, it is close to Narita, so probably enjoyable...