towen,
Your best bet is probably Fontani Camera at Viale F. Strozzi, 18 - 50129 Firenze. It's a fairly easy walk to the store from the San Lorenzo market area and also Firenze SMN (I know as I walked there last year at about this time). The store is not large but they have a great selection of camera gear, and as I recall they also have some "pre-owned" products at reasonable prices.
If you pull up Google Maps and click on Street View for "Gelateria Il Re Gelato", you'll see the Camera store (yellow awning) next to it. That will give you an idea what you're looking for. The Gelateria is an added bonus, as you can treat yourself to some Gelato and coffee after your purchase.
I also had a camera issue last year, and I was very pleased with the service at Fontani. I learned of them somewhat by accident, but that's another story.
Happy shopping!