craigslist, or I have to believe that in Boston/Cambridge/Somerville there are a few bike co-ops or other shops that will either have used bikes, or be a good resource to rebuild and maintain one you buy on CL.
I think mickey85's advice is good, get an 80's or 90's sport-touring bike that has decent clearance, many of the Miyata/Panasonic/Bridgestone/Schwinn/Nishiki/Panasonic/etc bikes of that era can fit pretty fat tires with fenders.
About 12 seconds of craigslist searching found these, which both seem serviceable, and in your price range:
http://boston.craigslist.org/gbs/bik/1769877324.html
http://boston.craigslist.org/nwb/bik/1769636885.html