Where do you go in Maine? I live here so I'm always curious about where visitors choose to tour!
I shop at Publix when I go to WDW. There might be cheaper options, but Publix is pretty affordable (compared to what we pay at home) and they have a HUGE variety of things, so it's one-stop shopping. We don't have the chain up here, but my daughter shops at Aldi regularly because the prices are so good. However, she says that produce is hit-or-miss, and you never know what grocery products will be in stock- but it's excellent for dairy/cheese, "dry" foods (pasta, crackers, snacks, etc) and canned/jarred things. I do not like Walmart produce- it always seems to rot almost immediately- and their meats (pork, beef) have a funky texture to me, so I don't usually buy groceries at Walmart. Goodings IS more expensive, but on our very first trip to WDW (back in 1998), when we have very young kids and knew nothing of Disney restaurants, we bought our suppers at Goodings as they had a large variety of ready-made meals, and at the time were the only grocery doing so. DH and BIL would take the girls back to the hotel (Residence Inn) and cook their supper while my sister and I went to Goodings and picked out individual meals for the adults. Not necessarily the cheapest option, but WAY more affordable than the Disney restaurants!
I would recommend Publix as an affordable, convenient, one-stop shopping trip at the start of vacation!