Masa Supplies on Cambridge Street
I found two great South American grocery stores on Cambridge Street. I don't remember the cross streets, but if you get off the Green Line at Lechmere and turn right at the candy factory, you'll find them. The first, Brother's Market, is actually on a small side street. I found Goya frozen banana leaves for $3.00.
A few blocks further down is the Pires Market, where I got a big bag of cal-boiled cornmeal for making masa. I still haven't found fresh masa in boston, but this is probably just as good. All I have to do is add water.