A week ago folks were saying they only found this at Whole Foods. I just confirmed that New Deal Fish Market (also carries sushi grade fish) and Courthouse on Cambridge St have this. Paid $2.99/lb with heads at Courthouse. Get some portuguese olives and paprika (colorau) while you are at it!
In Medford Captain Boston on Main St usually has it along with lots of sushi grade fish, but his prices tend to be a bit higher. I love Maine Shrimp, but think that Bread and Circus marks up a bit too much just for taking the heads off (which make great stock). I once saw them "previously frozen" for $7.99 and have never seen B&C selling for less than 6.99, no matter what the supply.
Out of curiosity do any fishmongers give away the heads anymore (assuming you buy something)? Courthouse used to, but now charges some.