The Portland Public Market (www.portlandmarket.com) is a good place, though not as good as it could/should be. I liked the cheese market.
Past Sebago Lake in the other direction, in Fryeburg, Maine (near Conway, NH), is a place called Weston's Farm (www.westonsfarm.com). I've bought some good stuff there.
I have friends who live in Sebago and have always sold blueberries from their field on the side of the road (GREAT blueberries -- cultivated, not wild, but big, dark and addictive, no pesticides). I think they are now selling garlic, either instead of or in addition to. I'm not sure how it works but if they're there it would be on Douglas Mountain Road going away from Sebago toward Baldwin/Cornish.
Is Brunswick too far north of Portland for you? We recently went to the Brunswick Farmer's Market, Friday mornings 9-12 on the common near the gazebo. Great selection of food - baked goods, fresh mushrooms, fresh veggies, goat cheese, local beef, etc. You can get a list by town with location and times off a website/Dept of Agriculture for Maine or pick up the brochure of Farm Stands at a Maine visitors center (we stopped at one on Rt 95). Some of the biggest vendors go to several markets.