The alleyways of Kenmore Square are excellent rat habitat. Pretty much the same as the alley behind Newbury Street. They're in and under the dumpsters. Basically anywhere food is served and there is a dumpster in the alley will work. It helps to have a dog with you to flush them out, got to be quick with the camera shutter.

Haymarket does have some of the finer rats in the city, nice and big with good looking fur.