Spencer, my information may be out of date, but Stereo Jacks and Looney Tunes are the names of two stores I carry around in my PDA. These are the directions I got from a thread some time ago:
Looney Tunes and Stereo Jacks
Drive up Mass Ave, and park at a meter when you reach Berklee College of Music. At the intersection there, to the left, is Looney Tunes. When done there, continue up Mass Ave, until it curves around (be careful, it's kinda tricky). On the left side about a mile down is Stereo Jacks. When done there, backtrack on Mass Ave, and park in Harvard Square. Find any music shop, and they will have a map of all of the record shops within walking distance (Planet records, Newbury comics, etc). That will take about a day.