Heards Pond in Wayland, i've seen carp jumping, and the Charles River has nice ones i've heard, and the Sudbury/Assabet river has them i'm not sure which one though it is. I don't know exactly where you mean by Eastern Mass though. I can tell you that places that are southern and not by the ocean are probably going to have a better chance of having carp.
Mystic river, more than you can shake a stick at. Stand on the rt. 28 bridge and you can look down and see where they are hanging out. All the way upriver at the lake part around the dam they travel in giant pods and look like a fleet of submarines. They are spooky and smart, so be stealthy.
The inner canals, they'll be jumping out of the water doing their mating thingy. A hair rig increases hookups, it makes it harder for them to inhale and spit it. 
The upper part of the Charles River (as in before the Waltham/Watertown dams) has a decent carp population. Only thing is that fishing those areas in summer can get tough due to the abundant weed growth. Fairhaven bay on the Sudbury River seems to be teeming with them, as we saw quite a few in the shallow areas in the summer. There are a few walking trails in the area that lead to decent shore access spots. I have also heard that Farm Pond has quite a few big ones, but I wouldn’t know how the shore access for them would be over there.
