Well, Rome is not really cheap.
The most convenient shop in Italy is generally Amazon.
However, you may find some good offers around.
My suggestion are to look for the camera you want to by on this website: trovaprezzi.it
Some of the shops listed are in Rome or at least can ship to Rome (just make sure you stay there a few days, depending on how fast is the delivery).
Also, you can try to find a local chain shop.
Most popular in Italy are:
If you speak Spanish (as I assume) it should not be too much a problem to read those websites.
Eventually, when you are closer to your trip, drop me a private message and I will try to help, even if I am not based in Rome.