Hi Paul,
I used to live in Kumamoto (for 4 years) and I can tell you right now that there isn't much choice. In fact, I did most of my film and chemical shopping in Fukuoka (usually at Yodobashi) since pickings were pretty slim. That said, towards the north end of Kamitori shopping arcade, next to Muji, is a
small camera store (second hand shop) that has a small cooler with film (35mm and 120, colour and black and white). They also did a same-day 3-hour turnaround for slide film developing, any size. I can't even get that here in Kyoto! I don't know if they still offer that service, but this is probably your best bet in the downtown area. They don't speak much English there, but they were always very nice to me whenever I went there.
As for Hard-off and 2nd Street, I used to visit those kinds of shops all the time and never once found a camera (other than maybe a simple point and shoot once?) there. You could get lucky but I wouldn't waste my time on it.
It's funny, I have an old friend from Kumamoto visiting me here in Kyoto right now, and he was telling me how much the city has changed since the earthquake (I haven't been back since it happened). So it's possible that some things have changed - but hopefully not too much!