where’s a good place to buy them in Cambridge, MA?
If you do not have seperate rims I would suggest direct tire place like Town Fair Tire(out of city) that does free switchovers for the life of the tire. It really is a bargain as you get your non-winter tires rotated for free(yearly) and your snow mounted on/off.
I think DirectTire does this too.
I live in Ohio and I bought mine from TireRack on line. I got the tyres and new rims delivered to my home for a good price and little effort.