You can use www.trenitalia.com - but note that you only have to book in advance for the Freccia/IC part of any trip: in this case the fastest route is Freccia to Florence and R (Regionale) train - headed for Grosseto - from there, total travel time 3h33 e.g. 10:25 arr. 13:28. But at other times, you might have to go via Bologna, Florence, Pisa and Follonica which takes longer.
Note that you can't get a price for R trains more than 7 days in advance, so for May you will have to look for times in the next 7 days, using the same or similar day of the week.
Thank you :)