I've been picking up CO2 from Tolman's right by the Broadway Station T station. They can re-fill paintball canisters as well as the regular CO2 tanks.
Check their hours as they close pretty early (I think around 3 or 4 every day and noon on Saturdays, closed on Sundays).
Haven't be able to find anywhere else.
Thanks a lot. I tried googling but it give me far away places.