There are many 'health food' stores in the Boston area. You didn't say where exactly you'll be living (South Shore? North Shore? West?), but you'll find many places that sell 'natural' and organic foods.
I live north of Boston, so I'm more familiar with the stores in this area. There are several Wild Oats stores - Beverly Farms and Saugus - as well as a Trader Joes in Swampscott. Boston and the surrounding areas (particularly, Cambridge) have MANY, MANY health food markets.
The best news in this area is that many of the 'mainstream' markets (Stop and Shop, Shaws/Star Market) are now stocking organic veggies; whole grain breads, cereals, and pastas; LOTS of soy items. You'll have no problem finding healthy eating choices.
Farmer markets abound in this area in the summer and fall. There are lots of options for obtaining organic products from local farms in this area. Many of the larger farms offer co-op memberships that will allow you to partake of portions of the harvest throughout the growing season.
Please e-mail me if you would like more specific information on more store names, locations of farmer markets, and co-op farms.
Good luck and welcome to Massachusetts!

Janice - Thanks for the welcome! I'm moving to Reading, and unfortunately know very little about where everything is! (My fiancé grew up there, so he's going to have to teach me how to get to different places.)

I think that you'll love living in Reading! It is a lovely community that has wonderful grocery stores, great farm market stands in the summer, and accessibility to all the major routes to Boston (including train service).
One of the things you should plan to do is to go to Boston's Haymarket Square early on a weekend morning to take advantage of the fabulous buys at the fruit/vegetable market stands. The selections are incredible and the prices are great! There is a way to "do" Haymarket - bargaining is acceptable, and you shouldn't buy the first thing you see, etc. If you e-mail me when you get here, I can fill you in on some of the 'tricks of the trade'.
Haymarket also has small storefronts that sell all sorts of wonderfully exotic cheeses, grains, and ethnic foods. It makes for a great excursion and provides a wide variety of fruits and veggies for your healthy lifestyle.
Good luck!!