The Garment District is a great place to shop for used clothes. The are located in Kendall SQ/MIT area. 200 Broadway, Cambridge See their schedule online here .

Filenes Basement is in Boston, but easy to get to from Cambridge and well worth the visit.  Unbeleivable deals can be found on designer items that can't be found anywhere else. Just take the Red Line from any Cambridge stop to Downtown Crossing. There's an entrance to The Basement from the subway station.