Did you try Homebrew Emporium? Their store in Weymouth handles CO2 - although I don't know if they swap or fill - I've just seen the tanks in their shop.
I get mine filled in Plymouth off Route 3 at exit 5 - Standard Repair & Welding Supply, 5 Raffaele Road, Plymouth, MA 02360 - (508) 830-0567.
A little drive from Cambridge, but I know the refill and not exchange.