There's one on the Mall in front of the Air and Space Museum; there's one in Boston that covers much of the city; and someone here reported that there was one in (I believe) Maine.
Everything I need to know I learned through Googling.
The model of the solar system in the Boston area stretches far beyond Boston. The sun, Mercury, and Venus are on the grounds of the Museum of Science; Earth is at the nearby Royal Sonesta Hotel; Mars at the Cambridgeside Galleria (shopping mall); Jupiter at South Station; Saturn at the Cambridge Public Library; Uranus at the Jamaica Plain Branch of the Boston Public Library; Neptune at the Square One Mall in Saugus; finally to Pluto, at the Riverside mass transit station in Newton.
The Maine Solar System Model ("built by the people of Aroostook County") has the sun at Folsom Hall, University of Maine, Presque Isle.