What is your home airport?
Do this:
Go to Kayak or Google Flights
Enter
from YOUR HOME AIRPORT
To PSA
click on the box to include nearby airports so you will get results to nearby airports, certainly Florence (FLR), chi h is only 50 miles away from Pisa.
Pisa airport has more flights than Florence, but both airports are managed by the same company (Toscana Aeroporti) which has decided to have Pisa specialize mostly on low cost airlines like RyanAir or EasyJet, while Florence, which can handle few flights, specializes on traditional airlines, like AirFrance, ITA, Lufthansa, KLM, Iberia, and their affiliates. If you can find a flight to PSA that would be best, but otherwise from the US with a traditional airline you might have more options to FLR.
From Florence to Pisa you can take trains, buses, automobiles. If you need a rental car during your stay, then rent it at the airport where you land, if you don’t, use public transportation.
I’m sure there would be more options to fly to Rome Fiumicino (FCO), Italy’s largest airport, however unless the flight is hugely cheaper, I would probably choose Pisa or Florence, and save yourself time since you need at least 3.5 hours from Rome to Pisa, whether you go by train or car. Milan is too far (about 5 hours from Pisa), I wouldn’t fly there.