The best local green coffee is from George Howell at Terroir in Acton. (www.terroircoffee.com) It's expensive, but excellent quality. He does offer a good discount over his roasted coffee. UPS Ground will get you overnight delivery anywhere in the Boston area.
Only a subset of his selection is available green. However, his Guatemelan Huehuetenango Finca El Injerto is available green, and might be my favorite coffee in the whole world (at least this week).
Armeno sells green, but at ripoff prices. They only give you $2 off their roasted prices. Since you lose 15 - 20% by weight in roasting, this makes most of their green coffee *more* expensive than their roasted.
Polcari's in the North End used to have a generic "green coffee" available. Don't know if they still do. I would only consider it for emergencies, or perhaps for learning a new roaster.
The Invalsa guy in Newbury that "itauanas" referred to indeed only sells his own imported Bolivian. However, he's the only source for a number of home roasters (he lists six on his website) in the Boston area, so be sure to bookmark his site . He has a booth at the Topsfield Fair every year, and is a very interesting person to talk to.
He gave me a sample of his Bolivian coffee, which I'm afraid I didn't like at all - after such a nice chat, I was hoping that I would. (It was a roasted sample, not a green one, so the problem might simply have been that it was roasted way too dark for my tastes.)
There is an importer of Bolivian Coffee on the North Shore, which does sell direct:
I mostly buy from Sweet Maria's and you can get it pretty quickly via their Priority Mail flat rate boxes (note they prefer UPS strongly, but I have had decent luck with PM as well).
The Whole Foods at Fresh Pond used to sell them. You might try calling one of the specialty coffee roasters in the area, such as Armeno's in Northborough. However you should also consider the Internet. I get mine at Sweet Maria's (sweetmarias.com). They are in California and have a very wide range of green beans to choose among.