There's a woman (I'm pretty sure she's Korean) selling homemade cookies at the GreenFlea market on 77th and Columbus on Sundays. She's on your left if you enter the playground from the entrance that's across from Shake Shack.
Her cookies are on the less-sweet side, which suits me. I especially like her peanut butter cookies (she grinds the peanuts herself), and the ginger cookies (with pieces of crystallized ginger), both of which are nice and crisp-chewy. If you get these, be sure to store them apart from her other, more moist, cookies so that they stay crisp.
She has a selection of sugar-free and/or fat-free cookies where the sugar and fat isn't replaced by anything artificial. Just extra nuts and or dried fruit holding it all together, hippy-style. They're a little odd, but I like them.
She's very friendly and obviously puts a lot of care into her products. One gets the sense that she's honed her recipes over time and is making cookies exactly the way she likes them. I think she charges $5 for 3 cookies, but sometimes tosses in an extra.