The SITA bus station is just a couple of hundred meters from the Santa Maria Novella train station in Florence (the main station), where you can check schedules and buy tickets. They have a website, but it is not working at the moment so I am not going to frustrate you by posting the broken link (they seemed to have changed their domain name and have some big mess going on - we will update this page when the situation changes). SITA buses run inter-city routes mostly to the south (think Chianti) and east of Florence - if you want to head to Siena and other towns on the way, then SITA is probably a good choice. They also have some tourist itineraries (like a visit to "The Mall"). Image below courtesy of Google Maps. Address: Via Santa Caterina da Siena, 17. Phone: 055 219 383
Here is a update - you can find the PDF schedule for the SITA bus lines here. For Florence to Siena you want (preferably, since it is the fastest) the CORSE RAPIDE / EXPRESS SERVICE.