I was in Nagasaki last month but didn't make it to Sasebo (partly due to poor planning on my part, as I had hoped to go). I'm not sure what you'll be able to find in terms of camera/film shops down there as neither Bic nor Yodobashi have stores there (however, if you are passing through Hakata (Fukuoka) Terminal, which I imagine you will be if you are taking the train, Yodobashi camera is just a minute walk from the Shinkansen-side exit of the station, and they have a good selection of film there). You may be able to find some hole-in-the-wall camera stores in the downtown area (that's been my experience in most Japanese cities), but there is some element of luck to that as well. Don't rely too heavily on Google or Google Maps to help you out - I've chased down stores (for film and/or camera gear) in lots of other cities in Japan and have always ended up empty-handed, as the stores were all permanently closed down. If you are really desperate, Kitamura Camera (a smaller chain everywhere in Japan) MIGHT have film, but most, if not all of it will be Fuji film, and probably just colour neg and 35mm (but sometimes they may have black and white. 120 is rare in those stores).
Nagasaki probably has more sites overall (Gunkanjima (Battleship Island), lots of places related to the foreign settlements - European, Chinese, Russian, etc., bomb memorials, etc.) than Sasebo, but I guess a lot depends on your photographic interests.
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