Purchasing a car seat in Florence

Seems like some car rental companies rent car seats.

I would feel confident in buying there - it will likely be more expensive than a cheapie here but will be easier not to deal with lugging it around before it is needed. Consider it a trip expense

You will be in Rome and Florence before getting the car - that is 2 big cities. I am sure you can find a baby supply store, but this will take research. The only thing I know offhand is prenatal. It's a chain babies-r-us like. I am not sure if the locations are easy to access without a car. Perhaps look around on their website - I am on mobile and don't have translate on so I can't quite read the details. https://www.prenatal.com

I'm sure there are smaller stores in the center as well.

rent it with the car.

There's a nice Carrefour hypermarket at Via Di Prato, 145 50041 CALENZANO a couple of km northwest of Sesto Fiorentino, out from the airport which probably has cheap ones. They may also be good quality; Carrefour doesn't sell much rubbish.

i could give you a bunch of places where you could buy car seats in Florence without the need to go all the way to the Carrefour all the way to Calenzano. There are plenty of places in Florence itself.
The issue is cost. A seat will cost you easily €100 and up. So I wonder if a booster would be ok. You can get a booster for under €30.
If you want a full seat, then the rental companies can add it to the rental for a fee.
Avis for example (just checked their website) charges 25 € per day (VAT incl.) up to a max of 3 days charged (therefore 75,00 € max) for a seat. For a booster they charge less than €9/day (also 3 days charged max=$26).
There might be discounts available.
There are also companies that rent it, probably at less, and deliver it to you. One I know is MammaMamma. Check their quote ("preventivo"=quote)
http://www.mammamamma.it/corner-mammamamma-home-uk.cfm
They have a shop in Rifredi district, not far from the airport, and they have used stuff too.

the biggest concern with having the rental car reserve is that they can not guarantee that they will have one when i get there. That means, having to wait at the Airport (which we aren't flying into, we are just picking up the car there) while my husband is off trying to find a carseat!!

I gave you above the name of a place that rents it and even delivers it to you at your hotel.
The Italian word for baby car seat is SEGGIOLINO AUTO
If you want to buy one, in Florence city center there is a baby product chain store, that is called "Chicco" (pronounce Kikko). It's Italy's largest company for baby products. I think they have them there. It's on Via Brunelleschi just north of the Piazza Della Repubblica. Basically looking at the Apple Store, turn right on the same street and walk.
https://www.chicco.it/in-auto/seggiolini-auto.html

If you rent your car at the airport, there is another specialized large chain store that sells those. It's in the Osmannoro district of the city of Sesto Fiorentino, practically almost walking distance from via Palagio degli Spini where the car rental center is. It's called "Prenatal" and is on Viale Redi (Sesto Fiorentino) across the street from the IKEA store. Actually there is even a Hertz rent a car office at IKEA, if you wish to rent from there. "Prenatal" sells baby car seats.
https://www.prenatal.com/it_it/viaggio-e-passeggio/auto-e-viaggio/seggiolini-auto.html

The above are specialized stores for baby products, however car seats are sold also in regular department stores. There might be even some of the Rinascente, on Piazza della Repubblica (across the piazza from the Apple Store) or at various auto parts stores, I'm sure. The issue is most of these places are not inside the historical center.