We are staying outside Florence central in a town near San Casciano. Is there a testing site which would work for us without going into central Florence?
What airport are you leaving from? Fiumincino has a test site. Pisa Airport has them in the morning, and Firenza airport does them - both use this: https://aeroporti.misecup.com/
This thread has a search site listed: https://community.ricksteves.com/travel-forum/italy/for-u-s-a-citizens-needing-a-test-to-go-back-to-your-home
You might ask your host if there is a local pharmacy that does testing
FLR has a testing center as well
https://www.aeroporto.firenze.it/en/the-passengers/covid-19/swab-test-reservation.html
If you are not already there consider bringing the proctored online self test
Lots of info in the COVID forum
https://community.ricksteves.com/travel-forum/covid19