Being a Nikonist myself, I can't recommend any shops for Fuji in particular, but for camera gear of any kind the place to go is Nipponbashi (日本橋) aka Den-Den Town ("Electric Town"), Osaka's equivalent to Tokyo's Akihabara, near Namba station. Tokiwa (トキワ) is the big name here, but there's a bunch of others as well.
Naniwa (ナニワ) near Shinsaibashi is also worth checking out, it's a large chain but their used-gear speciality shop (中古買取センター) is located here. There's another cluster near Umeda station with the Kitamura (キタムラ) chain's equivalent, used camera dealers Obayashi (大林), and some more Naniwa shops as well as well.
This map shows quite a few of these, but note that some shops shown (eg. Yodobashi and BIC) only sell new gear, and I think the one Naniwa outlet listed is gone. These don't see very many tourists, so expect English ability to be limited.