I just did this, so hopefully the info is fresh.

I went with TIM as they seem to have the best coverage in Italy. As a practical matter, I didn't leave Florence, so I could have gone with Vodafone, Wind or Tre (3) as well.

You need an unlocked phone (or whatever device), meaning it can't be SIM-locked to a carrier. Bring your passport, they'll use that number instead of a codice fiscale since you're not an Italian citizen (I presume). You will have to physically go to a store to do this, but at least in the cities, TIM is everywhere.

Searching Google Maps for stores in Italy isn't very good. You will probably want to use Google instead, and then map the address itself in Google Maps - although the placement isn't always perfect.

From there, I paid 12 euro for 200 mins, 200 SMS, 1GB data, good for 30 days. Plus the SIM itself was about 20 euro. That was with TIM.

If you are going to Florence and tell me where you're staying, I can tell you were the TIM store is in relation.

One more thing - if you plan to return, your SIM will expire unless reloaded every 12 months. You can do that online, so top it up once a year. It's cheaper than a new SIM.