Will be traveling in Italy in September. We have no reservations for anything as we are relying on a Space A flight but have no idea what day is available for us to fly. Thus, can't make reservations. How difficult will it be for us to find lodging, trains, and or rental cars? Itinerary is Venice, Florence, Tuscany.
Get the AirBnB and Hotel Tonight apps and that will take care of your lodging. If you are flying into a base rent a car from the Exchange concessionaire! I was going to do that (I'm former active Marine and my wife is with AAFES) but the same company that has the contract on base is at the airport. We'll be bouncing around Tuscany 9/5-16 - DM me if you're in the area during these dates!
Trains will be no problem. There are many and run frequently. No real need to have a reservation, you can buy tickets at the station as you go. I prefer to get them a day or so ahead so I am not stressed the day of travel. Some of the fast trains do have reserved seating, so make sure you check your ticket for the correct car.
Don't forget to validate your regional tickets or you will face fines!
Booking.com also has last-minute deals, your reservation will be confirmed immediately.
I would book on line as you go or as soon as you know and are there. Driving into town and looking sucks up a lot of time and in my experience (and I am terrible at this on the fly) you will waste time and get crummy places. Perhaps have a list of places you have pre-researched so you are ready to move when you first arrive and then line places up. Auto rental is more expensive doing it there and last minute but if you have a free flight that is one of the downsides.
Note that September is one of the most popular months to travel in Italy.
Both booking.com and venere.com will allow you to filter using your dates so you see only those places that are currently available.