google Boston Poly Groups
see what comes up.
There are groups on meetup.com as well.
The best way to meet potential partners, is to meet people. So, find a social activity that you enjoy.
The best way to avoid monogamous relationships, is to be clear with yourself and others that you are not interested in a monogamous relationship-no matter how horny you are or attractive they are. Period.
But, the best way to meet other like-minded people, is certainly to find group activities where those people socialize and that means finding other polys in your area.
If you have no luck on that front-try the lgbt groups and ask pointblank about poly groups through them.
I've found that there are a lot of poly's "hiding" in the lgbt community where we live.
"Love As Thou Wilt"