Ciao lolc,
I am not sure whether you're asking about a specific place called bagni caldi or whether you're interested in hot springs in general or in a specific area? There are lots of hot springs locations across Tuscany, from outside Lucca and Pisa to across the region toward the Val d'Orcia and to Monte Amiata area - take a look at this page with links to several:
https://www.discovertuscany.com/what-to-do-in-tuscany/thermal-baths/
This page has links to several OUTDOOR hot spring locations, from Saturnia to San Filippo.... but there are also many places that have the springs pass indoors through a Spa area so they are available year round!