I'm visiting Italy within the next few weeks and plan to use my phone mostly for directions and quickly finding the best buses/trains to get around. I'll be flying into Florence, so it would be great if there's an Italian mobile provider with prepaid SIM cards available at the airport. Might be even better to have a SIM card shipped to me before leaving the US, but I'll be leaving soon so there's not a whole lot of time for that. I plan to use an LG G3 VS985 and possibly a Moto G 3rd gen, HTC 10, or Galaxy S6; I'm pretty sure they'll all work in Europe.

Any advice appreciated!

http://prepaid-data-sim-card.wikia.com/wiki/Italy

The short version is that TIM has the best coverage while carriers like Wind have cheaper plans.

In the big cities, they all should have pretty good coverage but in the countryside, like maybe between Tuscan hill towns, TIM is likely to do better.

For a couple of weeks of use, if you are just using Google Maps here and there, you could probably get away with 1 GB of data.

I use a lot of data. Italy has one of the best prices for mobile data in Western Europe so I go for 5 or 10 GB. I often find that mobile data on TIM is often faster than wifi at many Italian hotels so I use it instead of the wifi, except for downloading a lot of podcasts or updating apps, which could burn data quickly.

I'm not familiar with your devices but typically, the Android phones they sell in the US can't get 4G or LTE outside the US. With iPhones sold in the last 2 years, you can get 4G or LTE in most parts of the world. So I got an iPad with cellular which is unlocked (meaning you can use any carrier's nano SIM card) and use a TIM nano SIM card. Then I can tether my laptop or my iPhone to it to get faster speed.

I've only been through Florence airport a couple of times. I don't believe they have mobile shops there. There might be convenience stores like Relay which sell some SIMs but you're better off going to a store associated with the carriers in Florence itself. You will need to take your passport and ask them to register the data bundle you want (SIM card and the data package you want are separate purchases). I would ask first to make sure the data bundle you choose is activated immediately. There are stories of people waiting 24 or 48 hours for the data bundle to become effective but I've never encountered that.

>>>here are stories of people waiting 24 or 48 hours for the data bundle to become effective but I've never encountered that.<<<

Or the first 24 hours uses at a higher cost.