The most important Etruscan necropolis in Tuscany are in Populonia and Vetulonia along the coast near Livorno and Grosseto but there are many, many others in the area. This area is referred to as the "Etruscan Coast" given the number of archaeological sites found and one of the parks is even called "Park of the Etruscans".
I've been to both, and thus recommend visiting both Vetulonia and Populonia if you're interested in the necropolis. They were major Etruscan cities and thus have several tombs sites. Here you can read much more details about each:
Another site I recommend is Roselle near Grosseto, a very interesting Etruscan archaeological area with an amphitheater, a large section of the walls that surrounded the town still standing, a few tombs and some mosaics still visible. Roselle is a pretty large site.