There are options for reaching Tuscany by bus but services are less frequent and considerably less comfortable than by train, considering both can take many long hours.
In the last few years, the choices that existed have diminished to presently saying there is Flixbus that takes you pretty much everywhere across Europe, Italy and Tuscany. If you're considering traveling to Tuscany by bus, take a look at FlixBus to see whether they depart from near you. Within Italy, there are buses from Rome, Venice and Milan to Florence and to other towns in Tuscany, just to name a few of the main Italian cities. There are connections from most European countries as well. The other bus company that works across Europe and Italy is Eurolines/Baltour but even they have started collaborating with Flixbus for some routes. Their biggest advantage are their ItalyPasses and Eurolines Passes, which for a set amount allow you to unlimited travel on their buses for a set amount of days (7, 15 or 30 days).
Consider Florence is used as a "hub" for transport, especially buses and trains. The central train station in Florence, Santa Maria Novella, is the main transportation hub in town, with many buses making stops nearby or at their own "stations" nearby.
You can consider using the bus for traveling within Tuscany as well since there are buses from Lucca, Siena, Grosseto, Cecina (along the coast), Pisa and others which are not easy to reach by train. So these recent growth of FlixBus, which has increased the destinations it reaches and competition to existing local bus companies has made it easy to find cheap bus tickets to and from Florence and to other towns in Tuscany, making traveling by bus a great option for reaching Tuscany, especially when you are on a tight budget.