Sad to say, but if you're dependent upon public transportation or taxis, Julians (http://jilliansboston.com/) is the best you can do.
Kings (http://kingsbackbay.com/) in the Back Bay area isn't too bad, it has gold crowns in decent shape. It is closer than Julians, but both of them are definitely night clubs first, pool rooms second.
There is also Flat Top Johnny's (http://www.flattopjohnnys.com/) in Cambridge which is a short ride on the Red Line away.
I think there is someplace down in China Town, but I'm not sure, that was a long time ago.
There was also a place on Washington Street when I worked in Boston, but again it wasn't really a pool hall.
There is also Boston Bowl (http://www.bostonbowl.com/) in the Dorchester section of Boston. It probably has the greatest action potential. It isn't within real walking distance of public transportation though and you'd have to take a cab. I guess you could take the T (subway) and a bus, but that is a pain in the butt.