Brunch/Lunch for locally sourced food?

Hi - will be visiting Boston in a few weeks time for a friends wedding. I will be staying at the Hotel Nine Zero and will not have a car - so I am looking at public transportation to get around for the weekend and I am not overly familiar with all the different neighborhoods.

The only day I will have free is Sunday - so, I'm looking for an excellent Brunch or Lunch place that uses locally sourced foods or sustainable practices.

Also, are there any food markets, farmers markets, etc that are worth a visit on a Sunday morning or afternoon?

Thanks for any help you can provide!