best site to purchase air tickets to Florence

Whoever gives you the best price. Use any of the general search engine sites for flights going there and then compare with purchasing directly from the airline. No one has magic pot of low airline fares. I generally prefer to purchase directly from the airline.

Check out kayak.com to easily research prices and itineraries from various airlines. Then you can either use the link thru that website to book from the airline you choose, or go directly to the airlines website.

Kayak is a good start. I also go directly to both Lufthansa and AirFrance or United. The fastest ways to Florence from SFO are with the above airlines (or their partners). If you can find a good deal try: SFO > FRA or MUC > FLR (Lufthansa/United) SFO > CDG > FLR (AirFrance/Delta code share) SFO > ZRH > FLR (Swiss - which is part of Lufthansa, therefore the Lufthansa 800 number might get you there that way). SFO > AMS > FLR (KLM/Alitalia/Delta code share). I don't like this solution because the layover in AMS is too long. In summer you can also try Pisa, which has a Delta seasonal flight to JFK: SFO > JFK > PSA (Delta) Via London you can fly British to Bologna (BLQ) SFO > LHR > BLQ From Bologna you can take the AppenninoShuttle bus to Florence (80 min) or bus+train to Florence. The high speed train ride from Bologna C.Le to Florence is only 37 min. and costs 25 euro one way. My preference goes to LH via Munich (MUC) or FRA. Or Swiss via ZRH.
I've done Air France to Florence several times, but dealing with CDG increases my already high blood pressure, especially when I have a short layover.